Minecraft Creepers made out of cardboard boxes. Made by painting directly on the box and gluing coloured / paper paper cut out into squares on top to make nice hard lines (not perfect but no-one noticed!) I didn't want to try and cut out the right shape, so i just glued black strips down the side to get the right shape. Used as a treasure hunt and then as a party game, lined up 3 of them and everyone took turns to see how many they could knock over with 3 balls at a time.
Minecraft sword made out of MDF then painted. Cut this out of wood and painted it ready to hit the pinata with. An Enderman pinata with a cardboard box so it is pretty sturdy and could put up with the sword for long enough for everyone to have a turn.