Toddler and Preschooler:

Funnels. That is it really. Just add funnels to your bath toys and the result is amazing! I bought three funnels in a set and they have had hours, and hours of fun with them. They have small, medium, and large.  They get a kick out of pouring cups of water into them and watching them go through. They also jus scoop water and watch it run out. They can put the funnels in a baby bottle and fill the bottle wth out spilling. -Try it!

You will need several inexpensive sponges and a bathtub.

Toddler and Preschooler:

Show your child how the sponges work.
They soak up a lot of water and can be squeezed out.
They can soak up water from a cup until it is empty and then be squeezed out to fill it up again.
They can float like a boat in the water.
They can be squeezed to pour water onto your body. (remember all those parts we talked about with the ball?) elbow, knees, etc and now add tummy and back, name the body part and squeeze water over that part.
Let your child find different ways to get water in and out of the sponges.
If you don't have a small basket for bath toys, start one now.