scratch board. Perhaps pre-make paint solution in disposable condiment containers? Take-and-go? Either not provide paint brushes, or require check-out (with drop cloth newspaper?) to contain potential disaster?
painting on paper mache- covered surfaces
Homemade Henna tattoos! 1 packet orange Kool-Aid, 1 drop green food coloring, 1 cup water. Paint on, let sit for 1 minute, pat dry. This could be fun.