Another fun thing to do, is hang out by the tall apartment buildings near the airport circle. These buildings are on the far end of the circle. Land a chopper up there, get out and then use a heavy sniper to snipe anyone trying to fly out of the airport. It's a lot of fun and people will have a really hard time getting you since they can't see you, but you can see them when they're in a vehicle and of course the only way they can get to you is by vehicle.
The only way to lose if you can't shoot whoever is coming after you out of the sky before they get to you.