Okay, let's for a second just ignore all the problems with time travel: the reasons it couldn't happen, all the paradox stuff. Let's assume it's totally possible and you can go there without affecting any timelines or screwing with history. Where would you go? What would you see? What historical events would you want to witness?
Man, it's such a big question. Personally I have two distinct things I'd like to do. To begin with, I'd want to go back in time. All the way back to the dinosaurs. I know -- this is about as cliche as it gets, but gosh darnit I want to see dinosaurs. I have no idea what specific period, but I think I would just jump around all the different times and see as many dinosaurs as possible.
Then I would like to go into the distant future, like 1000 years into the future. What does the world look like? What does technology look like? What do we as a human race look like? Man. I want to time travel so bad.
Anyway, the question is this: time travel. Where do you go and when?