Fall several thousand feet into water on the Liberty City Mission on GTA San andreas

this trick will let you partially explore Liberty City until you fall into oblivion

On the mission where you go to liberty city, continue the level until you reach the outside area where the main guy you've gotta kill is kill his friends but leave him alive (so you don't pass the mission)

Now do the High Jumps Cheat( Up, Up, Triangle, Triangle, Up, Up, Left, Right, Square, R2, R2)

now there are probably a number of places to do this so just jump around until you find a ledge or building you can climb over, now walk around at your own risk.
Some buildings and things are solid but the streets are not so if you walk onto them you will fall what seems like several hundred feet into a pool of water
(note that there isn't any escape besides loading or using the suicide cheats)