History

Maybe some math. People were really interested in tracking movement stars, planets etc. So if you learned their system of notation you might be able to speed up the development of certain mathematics since they'd see the "practical " value in it for astrological or religious purposes.

Edit: just realized Mark Twain's Connecticut Yankee is pretty much an answer to this question. Not sure it holds up to materialist analysis though.

I don't know how true this is but I think a lot of seafaring cultures didn't understand how you can sail into the wind (tacking). I mean you could probably go back in time thousands of years and show folks how to add a keel and how to point your sail correctly, no real "tech" needed.

just binge-watch Cody's Lab first

Going back to the early fifteenth century to tell indigenous people worldwide to use fire arrows against every European ship they see and to organize regular patrols to keep an eye on the ocean at all times. You can also inoculate people against smallpox just by sticking dried smallpox sores into their bloodstream. (The Chinese, Turks, and Africans already know about this at the time.) These two cool techniques could save tens of millions of lives and destroy the historical nightmare the world has been experiencing for the last five centuries before it even begins. Major issues are: organizing people and learning their languages.

Inoculation is an easy one if I could get someone to believe it, as well as whatever basic medical knowledge I have on hand (cool it with the bloodletting maybe?). Might be able to make a simple battery if I thought hard enough.

Could do a crude musket if I could figure out gunpowder (can remember saltpeter and ash(?) off the top of my head). That opens up cannons too. Assuming I've traveled to before these were readily available, whoever I swear allegiance to could have access to briefly unmatched firepower until the technology was copied. Crossbow should be possible as well, and also doesn't need as valuable of resources

Outside of the 20th and 21st century, I think most things should be replicable pretty far back, like into the classical age if you're somewhere fairly "developed" like Rome. It's hard to underestimate the importance of simply knowing that something is possible because it was done before.

probably the biggest impact i might be able to pull off would be some kind of irrigation improvement. like wind powered and/or muscle assisted (treadle-style maybe) pumping and storing at some moderate increase in elevation (~4 feet above crops) to a clay sealed pond. something i could pull off at a small scale as a proof of concept and communicate to others through demonstration for a larger scale project. besides slavery and shoulder buckets, flood and furrow was The Show for a long time, but it uses so much goddam water and salinity buildup collapsed whole civilizations. to be able to go deeper, have better flood protection, and build up an elevated reserve that could be more tightly controlled without a ton of labor time would probably be a major game changer in freeing up effort and protecting water resources, not to mention letting people farm reliably further away from rivers that are contested resources.

also, probably something related to plowing. like basically getting people to not do it when it isn't necessary, to protect top soil instead of just hammer it until it's gone and dampen the "need" to expand into new lands, find new sources of fertility, etc. probably a lighter footprint plow/more of a cultivator, nitrogen fixing rotations, cover crops and conservation strips to prevent erosion. that might be harder to convince, because the science has been definitive for decades and people still go hard on tillage and absolutely kick the shit out of soils because they think it is cool and makes fields look "tidy".

the easiest thing that would probably change history would be to go back to pre-antiquity and bring knowledge of the location and size of massive, easily accessible salt deposits and how to navigate there by the stars.