Then we made the simplest (and cheapest) robots imaginable. We attached marker pens to cups and put a DC motor and double AA battery on top. The "art robot" then drew some funny patterns on a large piece of paper. By weighting the cup differently we were able to vary our art style from pointillism to cubism. My robot was named Matisse.
To keep this super cheap I made sure everyone gave my markers back before they left.