Flip on a game and let it run overnight. Personally, I like Unreal Tournament set on a server game as a spectator. I pick one that has 32 players in it, turn off the monitor, and go to bed.
If there's some kind of loop demo for Medal of Honor or some other fairly new game, you might give that a shot instead. Quake3 has a time demo, but I wouldn't call Q3 that 'intensive' even with a P2-450 and a voodoo2 card.