Well, there's a lot:
1. I'd like to check out some dinosaurs.
2. I'd like to see the great pyramids of Giza being constructed, and the final product.
3. I'd like to visit ancient Greece and Rome. Maybe catch the murder of Julius Caesar.
4. I'd like to be around the time of Jesus. Maybe witness one of his miracles, and find out how close he and Mary Magdalene really were.
5. I'd like to visit some of my ancestors, because there is a big debate about the genealogy on my father's father side back around the 1750's.
6. I'd like to be there on the grassy knoll the day Kennedy was shot (not to see him die, but to maybe gather some new info).
7. I'd like to go back in time and make some investments in the stock market.
8. There are a bunch of Zeppelin concerts I would like to go to, starting with the first. Of course I would have to take today's recording technology, so we would have a few more bootlegs.
9. I'd like to go back and ignore the girl who became my wife.
As much as I would like to see Bonzo and some other rock stars not come to an early end to their lives, I wouldn't want to change anything that affects other people (except #9).