Shaving cream: kid fun + clean bathtub
Get yourself a cheap canister of shaving foam. Make sure it is non-toxic, although I have yet to see any that is toxic (durr. It goes on your face, so toxicity would be a pretty major design flaw). Get yourself some waterpaint and some squeeze bottles. Put a chip of watercolor paint in the squeeze bottle with warm water and shake until you have some funky colored water in there. Do not use food coloring, or you will likely be cleaning your grout and cursing a lot. [Melissa! Note dire grout warning! -- Ed.]
Now toss your toddler in the shower with the canister of shaving cream and the watercolor squeeze bottles. Make sure s/he knows how to use both. [i.e. not in the eyes. -- Ed.] Spraying the foam into strange shapes is half the fun. The other half is then dyeing the shapes and mixing colors.
If they are anything like my kid, they will be totally absorbed in creative wackiness for at least 20 minutes. Then you just turn on the shower with them in it, and zoop! Clean kid, clean shower. It’s like it never happened.
(Lisa suggests avoiding the menthol kind, and I want to remind you that shaving cream canisters leave rust rings when forgotten on the tub ledge.)