Maybe I will look into this stuff after all. From what I'm reading it seems that there are a number of interiors you can reach with the jetpack that you can reach no other way. Including Liberty City! It looks like credit for the find is going to Edison Carter, who is kind enough to divulge his secrets. It seems that these interior environments were created for cutscenes in the game. You may need Action Replay or Gameshark to reach some areas.
Edison Carter also discovered some brand new cheats. Cars Fly. All Traffic is Fast (Sports) Cars. Full Weapon Aiming While Driving. Nice job.
Two links above changed. Linked directly to Edison Carter's site to replace. –LL 7/21/07
I'm going to just laundry list a bunch of links here for my own reference, and anyone else who happens by.
Hidden Interiors thru the Gym!, w/ NO CHEAT (except jetpack)
Travels in and around San Andreas.
Found a way to fly jetpack around St Mark's Bistro. Dead link. So here's a different one. Added July 21, 2007.GTA/SA: Liberty City Tourist Guide
SA Hidden Interiors. This was a dead link. Found file elsewhere and changed the URL July 21, 2007.
Note: No page on my blog receives more hits than this one. So I got rid of the dead links. And I changed all references of “edisoncarter” to “Edison Carter”, that seems to be the name he calls himself over at his site. Enjoy. –LL, 7/21/07