I'd go to the year 2525 [http://www.youtube.com/watch?v=izQB2-Kmiic], see if man is still alive, see if woman has survived...
No really I'd go back in time and get samples of extinct animals and bring them back as a public service. Feel bad we killed the dodo, the Tasmanian tiger, passenger pigeon? Well I'll go ahead and fix that!
Then I'd go forward in time and bring back samples of advanced technology. You know, hovercars, personal genetic-recombinatiors, nanoforges, matter-energy conversion reactors and the like. Why? If you have to ask that question, then you don't understand.