One of the favorite bath activities in our house is shaving cream.
It's that simple--- just shaving cream.
I get the barbasol cans of shaving cream for 97c at Walmart and then just squirt in onto my kids hands during bath time. They LOVE it one can lasts for about 4 baths so it's cheap as well as easy
Plus clean-up is a snap!
My two year old prefers to smear it on herself
Or her baby sister, lol
While big brother (4 years) prefers to paint on the walls and the tub.
Sometimes for him, I'll just squirt a big pile of shaving cream onto the side of the tub, so he can grab it as needed.
I haven't tried it yet, but I'm sure it would be super fun with just a few drops of food dye also!