For my upcoming 18th birthday I've been planning a water party.  We don't have a pool, but we have several water guns, tons of water balloons, two places to attach hoses, and tubs and other large containers to put water in.  We accidentally started a water fight at my after graduation party, and we all loved it!  The thing is, it's a surprise.  Nobody ever comes dressed in anything but jeans and t-shirts so there's no dry-clean only things to ruin. You have to be cunning about starting it though. Splash someone with water and run aronud the house before escaping outside. That's what I do. ^_^  Most of my guests will be from ages 20-14, so I think everyone will enjoy it.  We're all 8 year olds at heart.