Group Brainstorming is always fun. (TIP: Get everyone one hopped up on sugar before.) Have a huge wall or blackboard (yes dry erase will work too) talk about the brainstorming topic and just let people freely and wildly say the first thing that comes to mind, give out chalk let them all have a blast doing it, see what wild imaginative people you have let your group feed off each others imagination. Sometime the best idea's come from the most off the wall brainstorming sessions.
So start brainstorming and see what you come up with.