Ghost World isn't as fun as the Interior Universe. There's always something new in the Interior Universe. Here's how you get to the Ghost World...
NOTE: MAKE SURE YOU HAVE FULL HEALTH. YOU MAY HAVE TO USE THE HEALTH CHEAT A FEW TIMES IN THE GHOST WORLD.
First, you HAVE to have finished all of Zero's missions in San Fierro (there are 3). If you have, go to the back room of Zero's RC Shop. There will be a red mission-marker there. Go into it to activate this side-mission. You are now in an RC Baron (plane) and you are on a roof. Fly west until you see the sea. Crash into the sea. Now, it will say that you have failed the mission; don't worry. In a few seconds, CJ will be falling and you will be in the Ghost World. DO NOT GO STRAIGHT FORWARD ONCE YOU'VE LANDED IN THE GHOST WORLD. GO ANOTHER DIRECTION.
Buildings and cars are invisible, but there are lots of people, and weapons and various other stuff are visible. Make sure you don't get into a dead end, though (where you can't move anywhere), and don't be afraid to use the Health/Armor cheat a lot (just don't save it if you're planning to get 100%). To get out of the Ghost World, go inside a building such as a gym or bar, etc. Buildings will not be visible on the map, you'll have to just know where a certain building is (where it's possible to get inside it, such as a gym or bar, etc).
Confusing?
South Park, Charmed, Buffy The Vampire Slayer / Angel, Tru Calling, Family Guy, TrippingTheRift, FreeForAll, Futurama, Jackass, Lost, That 70's Show, Celebrity Deathmatch