So lately I've been thinking about games I played at school and how these could translate into gameplay ideas for video games. It started with the water fight idea and kind of went from there. Of course, most of the games we played were either things that are already games, such as sports like football (though I don't think netball or rounders are games, I view this as a GOOD thing) or games which lacked enough of a structure to really make a good game (when we made them up, they tended to involve a loose narrative and a lot of running around aimlessly).
But there are some that would work. A game we called 'Blocky' has formed the basis for one of my ideas. It's basically a team stealth game, like capture the flag.
Maybe some others will come to mind in the future. It's quite a good way to think of game mechanics.