What would it be and what time period would you choose?
You can only choose one thing, though the amount of it is up to you, so long as its not like 500 tonnes of future spaghetti
or something.
I'd like to go back to the stone age with a bunch of fireworks and freak everybody the forget out.
You could probably come up with something better like bringing back some invention to make yourself rich or up humanities development, but I think messing with people would still be the most fun.
Paradox free theory time.