I’d want to hang out at the late-night jam sessions at tiny New York City jazz clubs in the '40s, when such virtuosi as Charlie Parker, Dizzy Gillespie, Theolonius Monk, Bud Powell, and Max Roach would get together and pretty much invent the jazz subgenre of bebop. Those after-hours sessions would probably contain more unbridled creativity and nonstop mad ideas than anything else I could think of.
I’d love to see Elvis live, preferably in his late-1950s rockabilly prime.
I saw the Duke Ellington Orchestra live in the mid-'90s, when Duke’s son Mercer (himself an old man, and since passed away) was the bandleader. But to see them back in their prime, maybe with Ella Fitzgerald as a special guest vocalist, would be a religious experience.
Frank Zappa, with one of those legendary bands of his. That would have to be awesome.
Morphine was one of my favorite bands ever (as a sax player myself), but singer/bassist Mark Sandman passed away a few years back. I always wished I could have seen them.
Louis Prima with Keely Smith and Sam Butera and the Witnesses, blowing the roof off a Las Vegas lounge at the height of their careers (the early '60s, perhaps).
The Ramones, preferably at CBGB’s in the late '70s.
Queen, while Freddie Mercury was still with us.
As for modern artists, I’d still love to see Tom Waits, Dick Dale, Brian Setzer (preferably with his swing Orchestra), Neko Case, U2, Tori Amos, Weezer, Nick Cave and the Bad Seeds, and Chris Isaak.
It would also be a real treat to go to a lounge/open mic setting to see William Shatner perform songs and spoken-word, accompanied by Ben Folds on piano.