Printing press. This was amazing, and the technology required for a basic version is virtually 0, it was just no-one had the idea first. For mass printing of something, say the bible, you can literally get a cylinder of metal with raised letters on it, cover it with ink and roll it down a page. Want to print 10,000 copies of the bible? Gonna be a lot quicker and more accurate with a bad printing press than by hand. This is widely credited as causing a dramatic increase in literacy rates, as there were a lot more books to read.

Weaponry. Forget guns, too hard. What is saltpeter anyway? Crossbows and pikes could be made easily by a time traveler and a couple of bowyers/blacksmiths. This made the poor significantly more even in combat against the rich (although I believe also led to people conscripting larger armies of farmers, and thus starvation) So maybe don't do this? If you wanted to make guns better, you would be best of creating a revolver (easy, all you need is a trigger that makes a hammer strike a bullet and rotates the chamber containing the bullets around. Easy enough for any blacksmith, and then when someone who knows chemistry comes along and discovers gunpowder, all they need to do is package it into bullets and you have advanced guns significantly.

Trains. Coal (I assume most people know of an abandoned coal mine?) gets burned, which heats water, which turns a fan, which turns an axle, which turns a wheel which runs along a metal track. Congratulations, you have revolutionised transport.

Science. Most people know enough science to help significantly. They would need to write down hundreds of facts that they learnt off the news, and run thousands of copies through the printing press they built. This is because 99% of these books will be burnt for heresy. After several of your scientific declarations are proved, people will start to take the rest seriously. Most people with a high school education would be able to; point out that light is a wave and a particle, if anyone doubts you, suggest they pass it through two slits and look at the interference, recommend pea plants as a good simple thing to look at when considering evolution and determining the difference between dominant and recessive traits, give scientists confidence that if they are looking for a particle that gives everything its mass, they just need to build a bigger circular ring and accelerate particles faster. Point out that matter is not made of plum puddings, point out that God does play dice with the universe, make the statement "lets assume that all observers see light at a constant speed relative to themselves, and thus time must dilate, length must contract, and mass must dilate." A mathematician would be able to go from there to relativity, relatively easily. Introduce the periodic table and fill it as far as you can remember. None of these would be useful immediately, but they would be extremely useful for the next 500 years.

OH&S - Give some dire warnings about the end of the world - asbestos, CFCs, CO₂ emissions. As you are now the most famous scientist ever, people will listen to your warnings, even though you are long dead.

Other engineering stuff I thought of now. Abacus (assuming you land in Europe and not somewhere it already exists), penny-farthing bicycle (no need for rubber wheels). There are a lot of inventions that you could vaguely discuss/draw picutres of, which would assist real inventors down the track. As with almost everything I have suggested, it would help years after you were dead. I'm trying to help humanity, not the time traveler. Trebuchet (just a catapult where the rock is in a sling attached to the end of the arm), the concept of arch/suspension bridges. Arguably, you could invent a relatively decent typewriter, although it might end up breaking too much.

Health - propose washing of hands, eating lemons to avoid scurvy, moving latrines further from food preparation, and recommend people investigate moldy bread to prevent infections. As with science, all of this will be ignored immediately, it just means that when one of these things is accepted, all the rest will be investigated.

Extremely modern stuff that you could describe, write about and draw, thus cause to be invented a bit early. The roller-ball mouse, airbags, dvorak keyboards, snuggie

Printing press. This was amazing, and the technology required for a basic version is virtually 0, it was just no-one had the idea first. For mass printing of something, say the bible, you can literally get a cylinder of metal with raised letters on it, cover it with ink and roll it down a page. Want to print 10,000 copies of the bible? Gonna be a lot quicker and more accurate with a bad printing press than by hand. This is widely credited as causing a dramatic increase in literacy rates, as there were a lot more books to read.

Weaponry. Forget guns, too hard. What is saltpeter anyway? Crossbows and pikes could be made easily by a time traveler and a couple of bowyers/blacksmiths. This made the poor significantly more even in combat against the rich (although I believe also led to people conscripting larger armies of farmers, and thus starvation) So maybe don't do this? If you wanted to make guns better, you would be best of creating a revolver (easy, all you need is a trigger that makes a hammer strike a bullet and rotates the chamber containing the bullets around. Easy enough for any blacksmith, and then when someone who knows chemistry comes along and discovers gunpowder, all they need to do is package it into bullets and you have advanced guns significantly.

Trains. Coal (I assume most people know of an abandoned coal mine?) gets burned, which heats water, which turns a fan, which turns an axle, which turns a wheel which runs along a metal track. Congratulations, you have revolutionised transport.

Science. Most people know enough science to help significantly. They would need to write down hundreds of facts that they learnt off the news, and run thousands of copies through the printing press they built. This is because 99% of these books will be burnt for heresy. After several of your scientific declarations are proved, people will start to take the rest seriously. Most people with a high school education would be able to; point out that light is a wave and a particle, if anyone doubts you, suggest they pass it through two slits and look at the interference, recommend pea plants as a good simple thing to look at when considering evolution and determining the difference between dominant and recessive traits, give scientists confidence that if they are looking for a particle that gives everything its mass, they just need to build a bigger circular ring and accelerate particles faster. Point out that matter is not made of plum puddings, point out that God does play dice with the universe, make the statement "lets assume that all observers see light at a constant speed relative to themselves, and thus time must dilate, length must contract, and mass must dilate." A mathematician would be able to go from there to relativity, relatively easily. Introduce the periodic table and fill it as far as you can remember. None of these would be useful immediately, but they would be extremely useful for the next 500 years.

OH&S - Give some dire warnings about the end of the world - asbestos, cfcs, CO2 emissions. As you are now the most famous scientist ever, people will listen to your warnings, even though you are long dead.