My daughter was dragging her feet about getting in the bathtub, so I told her I had an idea for a special surprise bath. I added about 2/3 cup of powdered milk and a heaping tablespoon of unsweetened cocoa to the water, and told her it was a "chocolate milk bath."
It looked pretty horrifying, but it smelled great. She spent the whole bathtime pretending that she was Augustus Gloop swimming in Willy Wonka's chocolate river! And it all rinsed out of the tub easily and didn't leave any residue on her body. (I did use the handheld shower to wash her hair, though.)