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.