You can do a mission, steal a car for Simeon, do a heist, rob stores, try out some races.... and there's loads more online content.
I've been mostly engaging in gang attacks this week.
When you do get a car for Simeon, usually someone gets pissed off, you get a mysterious call and find there's a bounty on your head. All other people in the session will see the bounty and likely come after you.
While all this is going on, Lester, or one of the other characters from the main game may phone you up to do a job, usually something like distracting the cops or brining down a smuggler plane.
There're a shit load of things to do but I must admit when I've had a crap day at work I just jump into my chopper and go looking for trouble.