I was totally in awe and inspired when I saw these "ART BOTS" over on Teacher Tom's blog. Oh my robot goodness. We did ours a little differently than his, you should definitely see how he made his for the kids in his school.
My son is a total tinker/builder/creator so we had an Erector Set with a motor in it. We simply taped the motor onto the top of a plastic cup and taped markers to the sides of the cup. I covered our kid table with paper and taped it down. Then we set the robot on the table and turned it on. It was so cool! My son was thrilled! There really isn't much for the kids to do once it is set loose on the table, but we had fun predicting what shapes and patterns it would create and if we moved it what would happen. It was a great experiment! We will be doing this often!