I should mention another thing I spend a lot of time with: making games glitch. The Deus Ex games are the main sources of fun in this respect; for example, I have spent many, many hours trying to get Paul to fall into the elevator shaft during the hotel fight, getting various people stuck inside doors (which is common to many games), getting the civilians in Trier scared of mice or using a vibrating corpse as a catapult to fling stuff around. The Thief, Jedi Knight and Splinter Cell games as well as most RTSs also have a number of possibilities for screwing around with the AI.
Actually, I did something rather cool with the collision physics in FS2 just a few days ago. I need to post a short video clip of this effect sometime.