I would say the dumbest thing I've done with a vehicle would be sinking the Chevy Blazer I had in college. Me and a friend moved into a duplex that had three hot chicks in the next unit, and he came up with the idea to go to the local river beach for a picnic (and see them in bikinis), which they surprisingly went along with. We could see some 4x4s doing light crossings down a little ways, so I figured hey I've got a truck lets do that!
Of course, the other guys were crossing at the shallow point. I on the other hand crossed at the deep point... Actually floated a little ways across and almost made it, but then hit a boulder with a tire which stopped my forward momentum, and slowly started sinking. The water was over the hood but not to the roof line, so we paid a guy $40 to get his backhoe and pull me out, then I had him drag me along the road in gear to blow water out of the engine.
She started up, and after about 20 minutes was running pretty much normal, and ran fine until I got rid of her. The bigger problem was that I had broken a kingpin so the front passenger wheel was always at an angle like "\" instead of "|". I didn't have the money to fix it so I got rid of her, and saw her on the lot fixed up a couple of weeks later.
That was the last off-road vehicle I owned up until this beater XJ that I just inherited. It's what my baby brother learned to drive on, and it's got some stories too (ie, losing the driveshaft on a powerline trail).