GTA .... build up a large knot of jacked vehicles, then set off an explosive chain reaction...
GT ... drive around the track in the wrong direction repeatedly, bashing into the other cars as hard and fast as possible. They punt into the air. It's strangely addictive.
Also spend far too long tuning one of the gearbox settings to give best possible top speed around the test track without taking an insane amount of time to get up to it or to recover from an off... and taking quite a while setting the other in a similar manner for each race, unless it's an easy win regardless of what I do (and the nitro strength-vs-longetivity setting, too). The most anal one for that is something really low powered on either the Nurburgring or Costa di Amalfi (either normal or reverse). Can you get a 2CV to both get up Kesselchen AND only just kiss the limiter going down the first blast after the start-finish line, and maintain at least 56mph on the back straight? Or to get up the CDA inclines and not rev out going the other way?
(yeah, first purchase is a set of sports tyres, nitrous, and a custom gearbox... everything else is just tweaks, but those each can make a massive difference unless you already have an engine powerful enough to spin the wheels in 4th without traction control, or a well-balanced race transmission... for some reason most of the "interesting" vehicles have shocking amounts of either under- or over-drive to the point where you wonder how they were even driveable in everyday life)
FPSes ... ROCKET JUMPS EVERYWHERE
Combat flight sims ... you'd better believe I'm going to try and get as high as possible, then dive and go as fast as possible, then make a break for the "border" at the edge of the map, assuming the makers haven't been smart enough to make that further than you could possibly cruise from any of the starting points. Also just shooting shit up for the sake of it rather than rigidly sticking to the mission plan. I'd be court-martialled so hard and so fast if I was actually in the air force...
Plus class-munchkining in RPGs that have class/job/skill etc systems. Trying to fill up the ball grids in FFX last time I got to play it... have everyone in FF5 try each job at least once... etc.