An educational bath toy that costs pennies to make: homemade siphon
My son is three and he loves water and the garden hose. So, I came up with a "hose-like" siphon he can use in the bathtub! Here's what I did:
I bought a four-foot length of plastic tubing at the hardware store. When it's time to play, I fill a large pitcher with water. Once my son's in the bath, I sit tubside and hold the pitcher. (The height of the edge of the tub is enough to let gravity work for the siphon. A higher counter will make the water flow even faster.)
My son starts the siphon by sucking on the tubing, and we have a working garden hose! As the pitcher starts to empty, I refill it with a cup and the tub water.
He loves to use his siphon to fill other cups and bowls in the tub, and meanwhile he's learning about gravity and water pressure.
When he's done, I hang the tube over the shower head to drip dry.
There's nothing better than a dead-simple toy/tool that invites hands-on (or mouth-on!) experimentation. My only suggestion would be to buy the tubing from the plumbing department and check that it's safe for drinking water.