Bathtub painting ranks high on my list of all-time best activities for toddlers and preschoolers. The idea is simple and brilliant: Soap-based paint enables your kid to be messy and get clean all at the same time.
There are companies that sell such paints, but don't bother buying them. You can make your own for pennies, using my foolproof and wonderfully imprecise recipe:
1. Combine liquid soap with a roughly equivalent amount of cornstarch; mix until smooth.
2. Pour into muffin tins or other small containers, then use a couple of drops of food coloring to create the paint colors you want.
This stuff is going on the skin of your precious little darling, so use something mild and reasonably free of carcinogens, endocrine disruptors, and sundry other scary toxins (you'd be surprised what's in seemingly innocuous products like Johnson & Johnson's Baby Bath).
The mixture is great for fingerpainting, if that's something your child enjoys. My kids are mysteriously fastidious about having paint on their hands, which I understand is fairly common among toddlers, so I usually give them brushes or sponges to use.
Your child will likely paint the bathtub walls and every accessible inch of his or her body (no, honey, you really shouldn't put paint in there). And most miraculously, from the parent's point of view, when you rinse everything off at the end, you will not only have given your child a great artistic and sensory experience: you will have cleaned the bathtub.