Bath painting is just shaving cream mixed with food coloring and the kids use this to paint our tub. So far we haven't had a problem with stains in the tub, although we did have some blue stains on our hands today so I think I'll try liquid watercolors next time.
In addition to the bath paints I found a recipe from Growing A Jeweled Rose called Goop and thought it would be fun to try. It's basically just a mixture of corn starch and water (and color, if you want) but the substance it makes is insane. It's liquid but also solid, when you try it you'll understand what I mean. Here's our Goop after I first mixed it.
At first Bea didn't want to play with it. She kept saying it was really sticky.