Our house usually had a theme called "Loggers Brawl" which we are thankfully bringing back next year. Lumber Jack theme inculding slided on the stairs and stuff like that. However, a couple people have scavenger hunts here and last year they came and were suppose to get the wierdest thing they could. One group of girls we gave a broken door that someone busted down, and another group we gave a broken microwave. Turns out they were form the same house and the microwave won. I'm sure that was a good sisterhood. We also had a Mud Bowl last year involving Alpha Phi. But they couldn't do it again this year after their advisor found out. That sucked. As you can see by the movie here:
Theta Xi - Alpha Phi Mud Bowl 2001
Its a great example of a dry party. Don't recommend a flour based mud though. Chocolate Pudding works well. You should definately get a hot tub as well, but be sure to spray people off thouroughly before getting into the tub.
Just some ideas.