My current favorite pastime is the Cargobob.
Like most, I wasn't the biggest fan of choppers at first. I still can't aim for shit with the mounted miniguns on the Scout Chopper (and unless it gets patched, I believe it to be quite an improbable, if not wholly impossible feat, respectively skill to develop), but aside that, I've become quite the pro at flying GTA V's helicopters.
Nothing quite like snagging a bus full of people up from the streets of Los Santos and then dropping them to their doom. I also made a game of grabbing driving vehicles off the freeways by matching my speed with theirs. The maneuver is a lot easier to perform on trucks than on cars, but hell - it's just the bigger payload to drop onto something. Or smack dudes in the face with, wrecking ball-style.
One of these days I'll have to snag up an armored transport. Roleplaying a stick-up man at its finest.
So - how do you make your own fun in GTA V?