Just a few ideas if you have a time machine laying around.
  1. Assassinate Hitler (obviously)
  2. Take your iPhone with you and take pictures of people. Show them the picture and tell them you've captured their soul in your "Magic God Box".
  3. Screenshot the aliens in Avatar, print it out, and tell people we get invaded by human-hybrid aliens in the future.
  4. Bring a picture of yourself, and right before they carve Mt. Rushmore...switch it with Lincoln's picture. Then tell your friends you're here to fulfill a prophecy.
  5. Bring a pair of crotchless panties with you and convince people that women in the future pee while walking.
  6. Go back before Trump was born and distribute his picture out to churches as the Antichrist.