realistically speaking, i'd be too afraid to go back in time and mess something up (if that's even possible... im a little shady on my meta-physics and space-time theory.) and of course if it was possible I would try to prevent major catastrophies of course.

Aside from that, I would go back to Jesus' time, and prolly go back to when i was 14 and tell my self how to train and eat ...

Gosh, there would be sooo many things to see and do, I wouldn't mind going back to the time of dinosaurs just to see them, and maybe going back to the 1800s and do some exploring and maybe some tiger hunting in Asia (don't flame me, I'm not a ruthless killer, I just love hunting and back then there were lots of tiger problems in some places )