Relive the first 3 years of high school, I was the biggest idiot that you could ever meet.
I'd also go to the date of creation for every big website in existence so that I could get decent usernames.

I would probably give myself winning lottery numbers, and I'd go into the future to find the cure for cancer, and other cool inventions, and bring them back into the past to save people's lives. Since bringing those items into the past would change the future, the future would have cooler inventions lol.