If you want to access Liberty City again, (you can just see the hill up to Marcos Bistro and I little of Portland View) you need the Gameshark/AR2/ARMax cheat for unlimited jetpack height. Enter that cheat and turn the game on. Go into the Ganton Gym and kill all people in there. Go and face the wall with the door on it (Behind the gym bags) and enter the spawn jetpack code (< > L1+L2+R1+R2 /\ \/ < >) and turn so you're facing the US flag on the wall. Walk to it about 2 steps and then thrust upwards. You will fly above the gym into nothingness. Pause the game and zoom the map. From where your guy is, go to the East Beach main road. Put a target there and fly there. Once you're over the marker, thrust down with square. You'll land on the reflective surface. Go foward a little until you see a little marker. Take off the pack and you'll get to a warehouse with no roof. Enter the jetpack code and fly upwards. Pause again and move the marker to the grassy area in Vinewood that's right next to Richman and Rodeo. Unpause and go up far (Hold X for about 1 minute) then turn CJ sideways. Thrust with R2 to the marker. The screen will be a weird color (depending on the time in the game) The hill will start to fade in. Be sure to bring a camera. Anyway, the only solid part is thewhole hill and the four way intersection. Into Hepburn, Chinatown, Portland View, will drop you to Earth. None of the buildings besides the bistro are solid. IF you want a video on this, go to EdisonCarter's site, Maxbot GTA Cheats He is the guy who supplies all those wacky button cheats to GTAForums.
EDIT: Also, in Liberty City's heaven (because it's about 3000m in the air, there's Hyman stadium from Vice City with stadium music. Why this is in the LC Interior, who the hell knows?