Great question! i wonder whether anyone has mentioned President Kennedy's assassination. There is a lot of controversy about it - I would like to see for myself. (I was in Dallas once and visited the site.)
Also, I'd like to witness Lincoln delivering the Gettysburg Address.
You said "witness," not "change," so I won't speculate any further.
PS It looks like JFK did come up already. Probably Lincoln too. Maybe I would like to witness a general place and time rather than an event. See my parents as they were when they were young. Visit a day in my own childhood or see some relatives who have died again, if I could stand the emotions it would stir up. Witness what an average day was like for my ancestors 1,000 years ago, or spend a day in ancient Rome.
Then again, we don't realize how hard life was for most of history, and how "boring" by today's standards. No internet, no mass communications, no grocery stores. Spending the whole day churning butter doesn't sound like much fun. But I suppose when people did go to church, or have a dance, or see a play it was a big deal. Maybe the quiet made them more contented and less stressed people somehow! Even the mass use of the internet is only 10 or 15 years old, and already it's hard to get through a day without goofing off here or looking something up.
Jesus appears to be by far the number one answer, and I would like to witness His life and ministry too. But for Christians we are supposed to already feel His presence and I hope I already do.
The future presents tremendous possibilities, as others have suggested. It's humbling to realize that we're all going to be gone sooner or later and the people of the future will probably look upon us as very primitive.
This is the best question I ever read here. You should get 1,000 extra points!