Use foaming hand soap in the bath
Foaming soap -- I think Kandoo makes a kid specific version, but I've just been using SoftSoap's -- is a great bath time hack. Why? Well, in one pump you don't use as much soap as you would with other liquid soaps, since it is both more watery and full of air. It's also a great "ingredient" for bath time play. Miss J. has been "cooking" with hers (a small cup and a plastic spoon) and is learning both her body parts and how to wash herself with it. I put a spot of foam on different body parts (knees, tummy, elbows, feet) and she washes/plays with it.