There's so much! Depends how bored I am. Actually, I rarely am. It's fun to take the girlfriends (especially Katie and Helena) on joy rides way out into the country (after you reach 100%) and watch them yell at you. Katie will climb onto the edge of a boat while it's moving, swim after you and even get into a helicopter!
Sometimes, a good old fistfight is just the thing, like with a hooker outside the hospital (they usually only carry knives), or with a pool player if he beats you at a game. Taking helicopter rides, plane rides or even go out in a boat, get an ATV and cruise around the back woods or up Mt. Chilead.
Practice flying the Hydra. Sneak onto one of the Army bases and steal the Patriot--see how far you get before they gun you down! You might make it home to your garage.
Base jumping and sky-diving are great too. Also, try the "Blue Hell" cheat or the trip to Liberty City (I've never gotten more than a few feet out of the alley before dropping through the wire frame).