Have you seen these fun bathtub markers from Crayola?
I recently went out on a limb and bought some for Cole. I feared he would spend the entire time drawing on himself or eating the markers, but he did surprisingly well and stuck (mostly) to coloring on the bathtub walls. :-)
And just for the record, I justified this totally unnecessary purchase by telling myself we could use the markers to practice colors (he still thinks every color is "pink") and writing his name. It's all about the justification! :-)