If the weather is warm enough, how about placing a kiddies' infatable paddling pool on a 'crash mat' of some kind and fill with water.
Rig a plank on boxes over the pool and have pairs of the guests beat seven colours of good-for-the-roses out of each other, etither with foam sword/clubs or feather pillows until one falls in.
Run it as a knock-out with some sort of escalating prize (or penalty) for eliminated at each level.

Walrus