I'd like to go back just to see what pre-feudalism was like, especially the bronze age civilisations and pre-Columbian America because I know very little about that.

In terms of actually changing history, give the Spartacists AK-47s or just tell the soviets not to fuck up the first invasion of Poland.

Or go waaaaay back, like before civilization, and drop some pack animals off in America so that the natives could build immunity to disease and make further use of domestication