Blind Cheap Booze Taste Test - Have a group of folks bring in a few cans of whatever weird cheap booze in a can that they can find at their local gas station (Colt 45, Old English, Steel Reserve, Buddha Beer, etc . . . ) Put them all behind a screen and serve people a taste of each without telling them what it is. (BONUS IDEA - Hide some fancy beer in there as well and see if people can tell)
Odds and Ends Game Jam - Bring a box of old sports odds and ends (Baseballs, rope, frisbees, etc . . .) Have a few groups try to come up with a game you can play with any combination of said stuff. Get people who want to try the resulting games
Ice Breaker - A first day way to meet new people. Something like a scavenger hunt, but for finding people. Or perhaps just a few low-stress games like word association, etc . . .
THE BEEG HIKE - Gather a posse. Find a destination to take a hike to, eat a picnic lunch there.
Music Jam - Gather all the musicians. Make them fight to the death, musically (or, you know, just play music together)
Something Ridiculously Formal - Some event with fancy dress and pomposity encouraged. Indubitably.
Fant Astic Yums Potluck - Have people bring something tasty for one evening after dinner. Try a bit of everything.
5 minute readings - Have a group bring in anything they can read to a group in under 5 minutes. Listen to what awesome things people have to say.