The person can go back in time only once, bringing up to 500 kg of supplies or materials with him. There he becomes immortal (but vulnerable), as the passage of time doesn't affect him anymore (because he traveled in time).
The bronze age society did not have the know-how or resources to distinguish decent quality imitation gems from the the real article. The time traveler could carry a lot of value in gems; they would not attract unwanted attention as anachronistic or magical---gems were normal goods that could easily find a buyer; and they could be hidden in a stash, and only sold in small batches.
The traveler could just carry good quality glass rhinestones, which are sufficiently advanced beyond local tech to not be recognized for what they are. Glass was known in late Bronze Age, but it was opaque soda glass; good clear glass only appeared in the Iron Age, and high refractive index flint or lead glass, which is what imitation gems are made of, much later. Rhinestones can be colored to match any real gem known to the ancients.
A much better quality of fake gem can be made of cubic zirconia. There is no way an ancient jeweler could tell it from a real gem, and it also can be tinted. It is more expensive than glass imitations, but much cheaper than actual gems. And its appearance could easily surpass the gems available in antiquity from India or the Urals.
A book of local plants and their herbal properties for the area.
Being a healer is great for your reputation, and this allows you to handle common injuries, ward off animals, and build a connection to rival tribes.
Knowledge of local ore deposits from archeology.
Finding good metal was always a challenge, and with detailed knowledge of local deposits you can bribe the locals to be nice to you.
Quality stainless steel blades.
They didn't have great quality metal back then. Well made steel blades and arrow heads and other sharp tools are a great trading tool, and are great for mugging people and taking their stuff. They were common trading goods as well.
Spices were fairly low mass, but very valuable back then. You could carry a decent amount of them to add to food and they'd make a valuable trading good.
Quality smithing, woodworking, architecture, and ship building.
These were all extremely valuable skills. If you wanna go back in time, consult an archeologist and learn how to make more advanced versions of local equipment and how to use them. You can bring some items to help, like thermometers, blades and measuring tools.
Whatever fragments of local language you can get.
Historical linguists have worked hard to work out as much of the old languages as they could. Whatever they've learned you can learn, so you can talk to the locals.
The ability to cure diseases is a huge edge, and a well attested to pill that could cure common medical issues would be a massive edge.
You can also bring back drugs to convert the natives.
Advanced modern crops for spices, drugs, and other products.
Once you bootstrap up, these can ensure a long term income for you. Make sure they fit the local area. Check with experts in the field.
Go back, with guns and drones and body armor and binoculars and laptops and solar power and other goods that will help you make a flashy first impression. Find local tribes that seem friendly, and try to make allies with gifts of medicine, drugs, and learn the local language. Perhaps you can pretend to be a god or a spirit. Once you feel secure, you can start selling your immensely valuable steel weapons for huge profits, and begin growing crops in the area.
In the long run, your electrical devices and guns and such will run out, but hopefully by then you've established yourself. You can then use your skills at crafting to make ships that can sail the seas with your goods, more metal tools, spice to trade, and solid buildings that can keep you safe, becoming a super rich merchant tycoon with allies who can ward off any attempt to kill you.
Similar to Ralf B's answer, yet more general.
Information -An atlas. -knowledge of natural disasters, the when the where the how bad -knowledge of other areas (who is going to invade in twenty years!) -knowledge of the location of rare resources -books about science and technology with schematics of inventions
For a very short while, the mere clothes on his back and his shoes will make him the richest being alive, because they are unique...
But as a time traveler traveling into a time in which food supply was more scarce, nobody speaks a modern language he speaks, and in which modern medicine and its tools are nonexistent, he will succumb to hunger and illness soon after. Typhoid Fever, Cholera, and Smallpox are among the more likely killers for him. In the short time that is left for him, he will be unable to communicate with the local people that all speak languages that don't resemble modern languages at all. Some tribes might also slay him for being an outsider.
My first instinct was to say "metallurgy" - with the right know-how, your protagonist could produce superior metals to the locals. Trading that skill to the right people in the right places for the right amount of time before moving on could allow the traveler to maintain a reasonably comfortable life (assuming they can avoid the hazards of life in the ancient world).
However... for true wealth, we need power and respect. While the priestly classes, in the right cultures, likely understood the patterns of the celestial sphere well enough to predict eclipses, and probably meteor showers, an outsider who could predict not only the same celestial motions as the local clerics but also knew when certain notable comets would appear - or, better, a supernova* - could have a leg up on them. Add in some strong powers of persuasion and such an individual could probably amass a fair amount of fear and respect among the locals, which could be leveraged into offerings, sacrifices, etc.
*: although even modern astronomers have trouble pinpointing when those happened without written records and a lot of extrapolation.
AmethystBrazilian emeralds would also have a good roi, but not as much.
Prior to discovering an enormous quantity of amethyst in South America, it was one of the five precious stones, with a value per gram on par with sapphire and diamond.
Today, it's a semiprecious stone with a value of $2-$10 per carat, roughly 1/1000th its ancient value. (Which still means that he can sink up to $25,000,000 on amethyst and remain under the weight limit.)
Taking natural, good-quality amethyst back in time would probably increase your wealth as much or more than anything else could.
The best way to leverage modern know-how in the Bronze Age would be to offer medical services based on modern knowledge, such as germ theory, blood circulation, antibiotics, anatomy, or endocrinology. The medical profession was always prestigious, earning respect and good compensation; a doctor that could actually cure ailments that others couldn't, could practically name his prices, and would be welcomed in royal courts.
Keep in mind, however, that the wide array of modern drugs, diagnostic tests, and treatments would not be available in the Bronze Age. The traveler would have to be his own pharmacist, using simple, available chemicals and herbs (penicillin can be produced naturally from the Penicillium mold); and his own diagnostic lab, examining patients by observation and palpation. The anesthetics that make modern surgery possible would not be available---the best alternative would be opium. But blood transfusion, donor to patient, could be done, leveraging the modern knowledge of blood types: blood type compatibility between donor and patient can be established with a simple improvised test. So simple thoracic surgery would be marginally possible, a higher-level advancement than Napoleonic-era surgery. The traveler should bring along a set of modern surgical tools, and a spare set or sets to be securely stashed.
In short, the traveler would benefit far more from the skillset of a 19th century physician and surgeon than a modern doctor's. And he would do best to adopt the attitude of Bronze Age Egyptian doctors, who examined the patient and stated up front: "This is a disease I will treat" or "This is an ailment that cannot be treated".
Aha! I get your question now. You could live forever from then on. Then the possibilities are manifold. Try surviving untill the first coins are made. Collect them all. Keep them all safe. The same for stamps. And van Goghs. A lot of art. Knowing beforehand will be so easy for getting rich!
Try to collect all stuff once cheap and now expensive. Take some collectables along from the bronze time. Take Roman art. Sll stuff to buy a gigantic storage nouse. Put everything collectable in there. Make full advantage of your knowledge. Will you change the course of time? Yes, but who cares? There is one thing though that must be done. Kill yourself. If not it will get pretty crowded in the past...
Since the time traveller can bring up to 500kg of things with him (per OP in the comments), I would suggest the following:
- bring a computer [or 2-3 for redundancy] (the simplest and easiest to repair, avoid modern superthin laptops) with the solar charger, spare parts (enough to make your modern equipment last for about 40-50 years);
- a digital database of human knowledge (similar to Wikipedia but with blueprints): Do not focus strictly on technology and 'hard' science, they are not the biggest money-makers;
- some gems to trade for initial capital;
- train in skills related to:
- administration and leadership,
- communication and applied psychology,
- minor repair of equipment he brings with him.
Perhaps, training similar to professional high-class spies would be appropriate since it would teach the time traveller how to blend in, how to get information, and so on.
Why all of these? Because, becoming rich is relatively easy, but staying rich and alive is not.
We often assume that a modern human will have some sort of superiority to ancient people and will easily become an existence capable of dominating the ancient world. This is how stories work. Unfortunately, plot armour does not exist in reality and any real time traveller would have to work hard to stay alive and protect their wealth (and life, too).
The time traveller should focus on establishing his status before attempting to become rich:
- modern societies value human lives and have various laws and procedures protecting them, in pre-modern societies outsiders must have power and status to protect their lives;
- only people with status are heard: A modern scientist may have a superior understanding of the world and its laws, however, without status, it will be very hard to use this knowledge or spread it;
- a specific status may be necessary to be able to own property and be rich: Bronze Age societies are not necessarily 'primitive', many of them had complicated social structures, property laws, and so on.
It is highly unrealistic that a nobody (the time traveller) with no connections, no family to rely on, and limited understanding of the local situation will have status qualifications to be rich. These qualifications will have to be obtained in one way or another. The time traveller should utilise his 'soft' skills to determine the safest and quickest way to become a part of the local society and raise his status. Combat skills should not be used to start a military career, but to protect his own life. A military career is too dangerous and may be a dead-end depending on the society, e.g. commoners are never allowed to become generals.
At this stage, it is important to determine what makes someone rich in this particular setting. Is it land ownership? Is it trade? Is it crafts? Is it religion? Depending on the society, it may turn out that the time traveller must become the ruler to be extremely rich and have his wealth secured. Please do not assume that our standards of wealth and social order apply to ancient societies. It is also unrealistic to think that one person can change society in just a few years. 'When in Rome, do as the Romans do' should become the time traveller's motto.
Once the time traveller has an appropriate status, he can sell part of his gems to obtain the initial capital. It is time to start using that database: Use it to develop solutions that fit the given setting.
If land ownership is the foundation of wealth, the time traveller should buy land and slowly implement better agricultural technologies.
If trade can make someone rich, start implementing modern business practices. In addition to that, the time traveller can use historical resource maps to find suitable locations for mines, new crops, etc.
If crafts can offer a path to wealth, the time traveller can build an industrial empire using the database and 'inventing' new materials and technologies.
The database will also be the foundation for long-term safety and survival. The time traveller should use it to raise and educate subordinates and various specialised personnel. Extreme wealth requires organisations of people. One person cannot manage big enterprises effectively.
No matter what path the time traveller chooses, he needs to make sure that he has enough power and develops military force and political connections to protect his wealth. Also, it is important to make sure that his wealth does not make the rulers too envious. Alternatively, he must prepare to rebel and fight for the throne.
This answer is based on an assumption that 'extremely rich' means 'comparable to the richest people in this setting'.
What is wealth, really?
The Bronze Age ran from 3300 BC to 1200 BC (according to Wikipedia). So, our modern notions of wealth are not going to be applicable: modern currency-based markets did not exist, no property registries, no banks (hell, they didn't have the digit zero until a millennium after the Bronze Age ended). So, what is wealth?
It's power. Power to direct the actions of people around you, power to acquire the best goods and materials for your own consumption. There really wouldn't be much you could do with hoarded wealth.
How do you gain power over a bunch of Bronze Age ignoramuses? Most easily, by convincing them you have supernatural powers.
This is precisely how the protagonist of Mark Twain's A Connecticut Yankee in King Arthur's Court does it: he uses his memory of eclipses to convince the locals he can predict the future, which he parlays into significant political power within the English monarchy. Then he uses his 19th-century knowledge of chemistry and mechanical engineering to establish a semi-modern enclave to mass-produce the kinds of early public works and consumer goods that earn him the loyalty of the population at large. Even simple stuff like quality nails is a huge deal.
So, as other posters have suggested, if you can bring back a lot of modern knowledge that you can study later, that's going to pay off, especially since you can't always predict what info you will need, and you may not be able to develop expertise in every relevant field before you go back in time. But, a good place to start would be the actual history of the relevant time and region, paying special attention to things you could make predictions about for purposes of impressing the natives.
You also want to protect this knowledge. Having it all written in modern English is a good start by itself, and on top of that even basic cryptography would likely be enough to prevent anyone but you from capitalizing on the info. Your goal is to ensure that you are the only person who can access the knowledge, because it's your insurance policy.
Rich people in the bronze age got that way by conquering other people and taking their stuff. Military strategy is way more sophisticated now than it was then, and students at military academies study old battles to understand what went down and why. This knowledge has practical application in the now.
A person with knowledge of strategy in the service of a halfway competent ruler could help the ruler conquer neighboring lands. Such a general would no doubt be richly rewarded. Don't get too greedy though or your patron might get suspicious of you.
Plus if you have to time travel naked (like you should) you can keep all of that learning safe in your head.
It would be pretty difficult to be extremely rich in the bronze age. There was no money yet besides metal rings, ribbones and axes. There wasnt a lot to buy yet, no real estate, no pieces of land no supermarkets, no technique, no most nowadays things. It is meaningless to ask how you could be superrich back then because you couldn't.
So the only real and true wealth is the experience if how a truly untouched form of pure paradisian Nature looks and feels like. Priceless, worth a fortune, and free. Something money can't buy anymore these days.
If there you should try to alter history so old Greece couldn't come into existence. It would amount to a very different world as that we live in these days.