John from our future, takes the time-travel bus back in time and steps off in 5th century Europe. He heads for the secret location where travellers are kitted out with authentic clothes, food and currency to keep them alive until they can establish themselves. He has learned enough of the local language to pass for a foreigner. He tells the locals he is from "Nonsylvania" or "far to the North" or such.
By time-travel law, he was not allowed to carry any artefact back with him and he didn't. He purchased a return ticket and and can use it by going back to the hideout according to the timetable. He is not allowed to make repeat journeys for any reason.
John is an entrepreneur. He left school early to work in a local market. He doesn't have scientific or detailed historical knowledge. There is no way he could invent even electric power. He doesn't know how to make rubber. If he wants to "invent" something that he knows about from his own century, he must do it with ancient tools and technology and get the locals to make it.
However John, contrary to all the rules of time-travel, wants to change the ancient world.
He settles on one simple invention from later than the 5th century that he can make and sell to build his business. He can employ local crafts people but they can only use their ancient tools.
My first thought was scissors but it turns out they were invented around 1500 BC in ancient Egypt.
I then thought of a can-opener (invented around 1810) but then he would have to invent cans. Given that John knows little about metals or where to find them, is it feasible that he could make this "invention" and set up his 5th century worldwide business empire?
Can he make cans? If he can't make cans, what can he make?
John has done no research before leaving. He is a businessman not a historian or scientist. His plan is to look around when he gets there and see if they are missing any everyday objects that he knows about. He only has to have a rough idea how to make them and maybe draw a sketch for the local blacksmith to try and emulate. He must rely on local skills, tools and knowledge to do the actual manufacturing.
By tool I mean a solid object, e.g. scissors, paperweight, hammer, axe, etc. Remember that John was not allowed to take any artefacts back in time (and didn't). He has to work from memory. Hence he will probably pick something simple but very useful.
Detailed timeline of inventions from Wikipedia https://en.wikipedia.org/wiki/Timeline_of_historic_inventions
EDIT - in defence of John
NOTE - What follows, superficially looks like story-line but it is actually an explanation of the conditions I set. Without clear conditions a question like this becomes far too open-ended.
Many people have queried John's lack of knowledge and research. John does not spend all his time on Worldbuilding, so he does not know about "black powder" and other arcane subjects. He is a man-in-the-street who made good. All his expertise lies in: making money, influencing people, spotting gaps in the market, organising teams to do the things he doesn't have the expertise for, etc. He was moderately rich and successful when he lived in his own century. Thus his skills are fully formed. Now that he can afford time-travel (it's very expensive), he is setting out on the adventure of his life. Instead of being a big fish in a medium-sized pond, he wants to be a big fish in the pond that is 5th century Earth. He didn't do research before starting because he isn't that kind of guy. He enjoys landing on his feet in a new situation and thinking fast. His intention is to spot gaps in the market - at first locally and then more and more widely. He will diversify when necessary and concentrate on efficient manufacturing and distribution - out-competing others when he can, and forming partnerships with them when he can't