Behind the prison, there is a parking lot. Take a large truck and back it up into the corner where 2 walls intersect (you'll see what I mean) Get up there and head toward the guard tower faceing the parking lot. After you get 6 stars it looks like a war zone with all the burnt out cars, bodies, and bullet holes.
Tw@ is also good if you like a bit of a challenge, stay tward the back and the cops will charge you.
Also, get a hummer or SUV and head for the beach at Firefly Island, and start shooting. I actually managed to get 2 helicopters to crash into each other, it was sweet. And it really brings out your inner hillbilly.
There are also some roofs right next to an overpass that are good if you feel like blowing up shit, shoot outs are my favorite GTA past time