Liquid nitrogen makes for some "cool" fun. A friend wanted to return some Tupperware. He was told they'd accept the stuff only if it was defective.
He dipped the stuff in liquid nitrogen and shattered all of it by dropping them on the floor. He returned the pieces in a bag and got his money back after he told them the stuff broke. He didn't tell them about the liquid nitrogen.
You can freeze a toy rubber pirate's knife in liquid nitrogen, then hammer it into a pieco of wood (desk, door, etc.).