We definately need to do this at iBoomerang:

Nerf guns. Everyone in the office is aware of their daily use, nobody ever aims for the face, and nearly everyone has one.

Post-lunch slump? Go to the office next door and “kill” a co-worker. The ensuing fire-fight pumps you up with adrenaline, makes you laugh and is loads of fun.

Within ten minutes everyone’s back at their desks, doing their work and having a great day. I love my job. :)

- Marizi

found in the comments on Making Work Fun via lifehacker