BomberClone, Scorched3D, Sauerbraten (typo?), GLTron and go down well here... Turbo Sliders, which my ex boss mocked went down a storm - top down single screen 2D racing a la the old Supersprint series but with more technique. Had a whole room playing tournaments on it, great fun - not very high resource requirements either. If you have higher specced machines, Gene Rally is very similar but with much better 3D style graphics...

There's also Kumoon which is very kiddie friendly but not multiplayer - strangely addictive. Worth a look even if it's just for you to kill time at breaks :P

Thanks for the heads up on the paintball - may go down better with staff than kids running round with shotguns and rocket launchers!