h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This one is a little more strenous than the others mentioned so far. You will need a minimum of two players for this game, but six or seven is much better. First, wait for it to snow. This may take several months depending on when you start the game. Once it has snowed, don suitable attire for the wintry scene. A pair of wollen gloves soak through particularly quickly in the snow, allowing you to fully experience the frost bitten fingers aspect of the game. Next, build two low snow walls, facing each other just within throwing distance. Collect as large an array of snowballs behind each wall as possible and arrange half of the players behind one wall and half behind the other. The outwardly visible objective of this game is to throw snowballs at the opposing players and attempt to knock down their wall. The actual objective however, is, after some undetermined time, to suddenly charge the opposing teams wall and kick it down. This is immensely satisfying in a strange primordial way. The actual signal to charge has never been accurately identified, but is a latent instinct, carried through from the last ice age and is instantly sublimily recognisable to all players. Sometimes players attempt a sneak attack by circling round to the side and hopefully springing out from the rear. The futility of attempting to circle round in a large yellow fleece and red scarf in a wide open field that has been conveniently whitened with a layer of snow, passes most people by in this display of primitive hunting techniques. Finally go home, for a change of clothes, a mug of hot tea and tell the kids how you totally vanquished their best friends dad.

See Grass Dens / Fights...
It works on a similar basis and was a huge favourite of mine.