Oooo.. spoilt for choice!
I would go back and watch as many battles as I could, and try to fit in a conversation with all the great generals, including those on the opposition.. assuming they could understand English.. babel fish anyone?
Then I would go into the future, see what's going to come of our existence..maybe a couple of concerts like Woodstock and such. Hang out in Parliaments to see through major decisions, re-live them.
... My tardis would look like a Lotus Exige but, of course, a lot bigger on the inside. I would do it up.. hmmm... like an apartment. Self-contained flat... could get a room-mate I suppose.. Oh, I would take my friends back to those awesome concerts as well... and maybe to the future, if I feel like it.