good friends, costumes, alcohol, cornstarch... how can you go wrong??!
not sure if it is the oil from everyone's hands or just the reaction it takes once it has been wet, but cornstarch is actually fair easy to clean up afterwords... it doesn't get all dusty, it just sweeps/vacuums up, and you can just hose down the backyard
DISCLAIMER: I DO NOT recommend bringing the cornstarch party into your house, but on the offside that you do? it will be something you and your friends will never forget :)