A nerf tag game, that can sometimes take up to a month.
What you do is get an assload of friends and make groups of two. After all the rules are set and (If you chose to, have everyone put in a dollar or two) the game has started, you continue leading a normal life, but if you see someone playing you can shoot them with a nerfgun and knock them out. The last team standing wins the pot of money. Two team mates don't need to be "in" for the victory to be valid.
Its more fun if you have a car and a drivers license. Also, its a good thing if you are "Safe" during school, to prevent all the trickery and silliness that goes on in the school yard.