What are technologies that a time traveler could easily introduce to ancient times?

[Eldan]

Antisepsis. If you can concentrate ethanol enough it should be potent enough.

Building on that: distillation. Very simple distillation should be doable even with bronze vessels and water cooling.

Thinking a bit further afield about things I know:

If we're early enough, crop rotation. That should be easy enough to explain.

Algebra? I mean, my high school education is a while ago, and I couldn't explain calculus to anyone (or even do much of it), but I could introduce arabic numerals and from there, a few simple things like quadratic equations or long division.