Driving Like an Idiot:
My favorite thing to do since III has been stealing cars and then driving around the city like a maniac. There has always been something thrilling to me about this. Avoiding (or not avoiding) pedestrians and other vehicles, jumping off of highways only to smash into other cars....oh, it's thrilling.
Explosions:
Ever since I got into Goldeneye, I've loved making stuff explode in video-games. If you recall those good ol' days, planting mines around the map and then waiting for someone unfortunate to walk by and causing them to die in a firey explosion, then you'll know what I mean. In GTA, one of my other favorite things to do (while more than likely having the invincible cheat on and unlimited ammo) was make a lot of things explode. I used to drive as many cars as possible to one area, then go up on a ledge and shoot a rocket at them. The cops would come in a hurry, and then it's waste them all with more rockets. Then I'd fly away in my jetpack.
Cheating:
One of my all time favorite things about GTA were the cheat codes. I'd turn on as many as possible at time to make the game extremely fun, and the combinations of some were just so ammusing you couldn't resist. Back in Vice City, I'd turn on the cheat that let you drive cars on water. I'd then do a lot of stuff to get police to chase me, then I'd drive into the water and watch with glee as the police drive into the water and drown...followed by an explosion. Another of my favorite things in Vice City was to stand on top of a box (Not sure what it was, it was years ago I did this) and have invincibilty on. I'd get my Wanted level as high as possible, then let them swarm me. I'd get shot thousands of times, but I'd still be perfectly fine. Then I'd steal a tank and run over people.