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.