I would probably travel back to the 1920s and make sure that Tesla's inventions came to fruition rather than being seized by the FBI upon his death never to be seen again. I truly believe his inventions would have changed the world as we know it.
I would probably try something similar. Tesla was genious that never really got enough credit. I also believe he had working free energy devices and other exotic stuff that would have basically liberated mankind from the current status quo.