There's so much to do in that game, probably my favorite open world game ever. But you have to unlock a good portion of content by completing some missions first.

My favorite pasttime is getting the two Grove Street gangs to fight each other. As Franklin, you travel to the blue gang neighborhood, start some shit, and slowly lure them down the street and across the intersection. Then the green gang automatically starts shooting at them, and chaos ensues.

Tailing cops is also fun. Usually they will eventually turn on their sirens and speed toward a car whose driver happens to have a gun, then they get into a shootout with them. Getting gangs to fight the cops is also a good time.