This needs to be moved to the History Discussion Forum - and I'm looking forward to reading the responses!
So many possibilities!! Of course, one of the challenges of this kind of exercise is making the choice, knowing that if you really were able to go back in time, you wouldn't have the benefit of the knowledge you have today about how events were going to play out and what problems would be faced along the way.
Nevertheless, if I had to make a choice of only one general area and time span, I would go to the Boston-Philadelphia corridor in the 1750-1800 time span to witness the interchange of ideas and influences that came together to invent a new country. Maybe I would be a barmaid at one of the taverns frequented by Jefferson, Adams, the Lees, and Franklin. Listening in to the debates and getting involved in the excitement of the revolutionary process would be exhilarating!