Some GTA is always fun, my two favorites being SanAndreas and GTA IV (along with its episodes).
I discovered an epic glitch in SA. Turn on the "Cars fly when hit" code and spawn a Hydra. Hit the Hydra with a car and quickly get in it, don't fly or else you'll screw it up. This causes the game to think the Hydra is still on the ground, allowing you to float past the flight boundries, which can help in finding hidden interiors. It also makes for epic sky diving.