Arthur in his Rubik's Cube costume, Halloween 2006, originally uploaded by Tané Tachyon.
This fun costume idea could be almost completely free! All you need is a big cube box with head and arm holes cut out of it. Paint the cube black and past nine different colored squares on each Rubik's cube face. You could paste the squares on randomly for a mixed up Rubik's cube, or a solved Rubik's cube.