Eva Cassidy, definitely.
I’d also like to go back to 1975 and see a performance of the British show “Hulla Baloo,” a musical set in a “loo” that featured “contributions by Tim Rice and Andrew Lloyd Webber.” It was so bad that neither of them mention it in their CV.
Then I would buy any souvenir of it and put them on ebay today and make some cash.
If I could turn back time?
Cher, obviously. 
(OK, actually CCR.)
–Cliffy
Charlie Parker
Miles Davis
Chet Baker
Stan Getz
Dizzie Gilespie
A host of other jazz greats
Queen (with Freddie)
Warren Zevon
John Coltrane
The Allman Brothers when Duane was still alive
Gram Parsons/The Flying Burrito Brothers
Thin Lizzy anytime in the mid to late 70’s
Lords of the New Church
KIX at any of their New Year’s Eve shows at Hammerjacks
Girl
Saturnus in 1996
At the Gates’
Curve
Eva Cassidy
Jeff Buckley
Allman Bros. w/ Duane and Barry
Robert Johnson
comedians: Bill Hicks, Sam Kinnison, Richard Pryor, Mitch Hedberg (still can’t believe he’s dead already)
And definitely Alice in Chains.
Might you happen to be near Jersey on August 12? If so, I’ll see ya there.
Well, you just completely stole my answer. For years I’ve been saying that were I to hit the lottery today, I’d pay whatever it took to get those guys to reunite for just one more show. I’ve seen them somewhere in the neighborhood of 40 times or so, and damn do I miss going to their shows.
I’d like to see my great-uncle Ted (who died before I was born) playing jazz clarinet with The Red Nichols Band. I’ve also been told he sat in on sessions with the Dorsey Brothers.
Bastard!
Okay, since that one is taken, lemme see… I guess it would have pretty swell to have seen
Florence Foster Jenkin’s legendary Carnegie Hall show.
Either that or to have seen the anonymous cave painter(s) at the Grotte de Lascaux.
You’re confusing The Firebird and The Rite of Spring
Of course, to travel back in time and watch great sporting events, you’d also need to wipe your memory first, as knowing how it turned out would kind of spoil things…
Unless you’re betting on the game, but that’s another thread entirely 
I kinda meant someone you could have seen, but didn’t for whatever reason.
But this is good. Better, even. 
You folks are stirring up memories, for sure.
Peace,
mangeorge
Fletcher Henderson’s band battling Jean Goldkette’s at Roseland Ballroom.
Duke Ellington the night he opened in London.
Benny Goodman on the “Let’s Dance” program.
Joe Haymes’ orchestra at the Steel Pier.
Prof. Theremin’s all-electronic ensemble.