Colored Ice Cubes
Drop colored ice cubes in the bathtub and watch your toddler chase them around before they melt, dying the water.
Fishing for Letters
With magnet ABC letters, a colored bathtub, and a fishing pole with a magnet tied to the end, your kid can go fishing in the bathroom!
This looks so fun, I WANT TO DO IT! Just visit your local dollar store, pick up some glow sticks and drop them in the tub. According to Itz Mitz, you can freeze them and retain their glow until your next usage!