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

[Accelerator]

A really simple one that people might not have heard of is a bellows gun, which is literally a bellows used like a suped up blow gun to fire a dart at 300 fps. They didn't get invented until the late 1500s but the underlying tech is much older, they just didn't put them together until gunpowder weapons made the shapes for them.

Wait. An air gun?