I like boats. I like bumper cars. Somehow, no one's thought to combine them. So, I present, ROWBOAT WARS!
You have a lake and every player gets a mini boat, or maybe two people to each boat. They could have engines, but that'd add to the cost and make the title void. Big bumpers make collisions more exciting, and rigid fibreglass hulls mean no one's going to capsize or sink. Each one also has a turret with a Super Soaker on it which sucks water up from the lake.