For the record, San Andreas is the only GTA game I like. The others are more 'These would be kinda fun to me, but not enough.', even though Niko Bellic seems an alright dude.
NOW THEN, the fun part. All the snickery stuff I do in GTA: San Andreas, because I never even bothered with the plot! (I'm sure the plot is good, but I bought the game to screw around.) Ahem...
* Driving a car so fast that I flip upside-down and slide down a hill (in the rain) to the tune of Horse With No Name.
* Tossing grenades/plastic-explosives/dynamite at cramped highways to watch the cars panic, funnier when they try to ram through all the dead hulks that're there already, and watch all the lovely cascading explosions caused by this.
* Burning people. Just burning people. Gangers come by? Burn. Someone makes fun of you? Burn. Looking funny? Burn.
* Using the drive-on-water cheat to get to blocked-off places. Or jet packs. Or an actual jet. It's technically more fun to do it the illegitimate way and explore.
* Driving a motorcycle around at maximum speed...just to see how amusingly I can wipe out. To date, nothing beats the time I struck part of a bridge and fell all the way down into a river.
* Stealing vehicles and driving them places at max speed so I can dive out and use their fiery selves as missles against stuff for when they're about to explode.
* So, one time I took a helicopter and flew it really high, because I didn't really know how to control the thing. Then...I accidentally got out, and while I was figuring out what to do, CJ went "WHAT THE FU-" SPLAT!
* Spawning a jet and...then watching it slide down the hill and - Whoops - explode.
* Driving a hovercraft or an ATV! Seriously, this is screwed up, yo!
* Setting fire to the inside of a bar (I'm invincible, BTW.) so that you can watch other people - who can't LEAVE - panic about the fire (or being ON fire).
* I also enjoy pool. And when I lose, I beat the other guy to death for cheating me.