Calculations. You could take a $2 solar calculator and make heaps of money just doing math for people. They used to have large tables of calculations predone, but these tables sometimes had errors and the like. Your calculator would be a lot more reliable, and quicker. A suitcase full of them would be worth a fortune.
Would (nonhistorical) textbooks be too Biff Tennon? If you went back to the age of the Greeks armed with books in English on how to cheaply, mine, and refine iron ore, plus how to make to steel, gun powder, steam power, find fossil fuels, and the plethora of modern knowledge you’d be the most powerful person in Greece because you’d be only person who speaks common modern English, and that’s what your books are in. Throw in some books on making penicillin and you’d be a minor god able to bring people back from certain death.
Also you and some friends could offer unbreakable coded communication. No one can read english there. If your friend is in one city state, and you in another you can openly send letters back and fourth no one can read, but you.
Doesn’t “modern, improved seeds” also imply sterility? That is, you grow a crop of corn from the cool seeds that let corn grow like crazy in crappy dirt, but you can’t take the resulting corn kernels and use them again next year to grow a new crop. They’re sterile, you have to buy new seeds from Monsanto.
I can’t imagine that going over very well in the middle ages.
I’ll suggest something that isn’t always suggested.
Take some nice clothing from today. Nothing super expensive would be needed to be rich. Four hundred years ago clothing was worth a lot. Add in a couple Kevlar suits for your protection and many for sale to the elite.
They are not sterile. At worst they are a hybrid that will produce the two original parents and those parents are still way better than corn from 400 years ago.
Depends on which “modern, improved seeds” you mean. The engineered sterility is an issue for some “products” of agribusiness but not for others.
It’s been said (by Jared Diamond) that growing grain is the single most profitable activity humanity has ever engaged in. A single wheat grain produces 200 grains in 110-130 days (germination to harvest time taken from Wikipedia). And wheat is relatively easy to cultivate – it can be sown by hand (scattered into furrows). No grafting, no repotting or extensive digging, no need to train vines or prune trees.
What else delivers 200 times return on investment in 4 months AND will keep you alive even if the market crashes?