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 spent all day reading about early suspensions on carriages and cars - it wasn't until the 16th century that they figured out you could make the ride smoother by suspending the body of a carriage with leather straps (as opposed to having it be attached directly to the frame), and it wasn't until the 19th that they figured out that you could make it even smoother with a leaf spring suspension. Leaf springs themselves actually date back to ancient times and can be made of metal or wood depending on what's available, so it's just a matter of applying them to a new purpose.

Depending on how far you get sent back, there's also a lot of very simple improvements you can make to wheels that were technologically possible for a long time before people thought to do it. Make wheels lighter by making them out of thin planks instead of an entire slice of tree trunk, stronger by reinforcing the rim with a metal band, more maneuverable by separating the axle into two sections. Inflatable tires are much harder, but people would use a solid band of rubber or cork around the outside of the wheel to accomplish the same thing before pneumatic tires were figured out.

If you're doing either of those things, then you're a stone's throw away from inventing a bicycle. Just a thought.

I would build an astronomical telescope way before those nerds in the 1600s

Math, some simple tools like 19th century plows, better crop rotation and artificial selection, 17th century and later metallurgy. Basic but useful electricity, a motor can be made from wire and magnets.

Much much better medicine. I can definitely recreate penicillin and gramicidin, ether, basic surgery. I can make small amounts of aluminium, also a map of accessible high yield ore deposits.

Once super rich steam engines and telegraphs and better ships become possible, but the trick is giving people what they actually need and avoiding the "man who came too early" scenario.

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.