Alex Toys manufactures some of the most interesting bath toys I have ever seen, and my favourite amongst these has to be the Water Flutes.
This little toy consists of five colour-coded plastic flutes held together with a dense foam dohickey. Each flute is designed to be filled with water; the height of the column of water inside the flute dictates the note that the flute will make. The clear plastic columns are calibrated, showing precisely how much water to add in order to achieve any note within one full octave (C to C). The mouthpiece of each flute is a different colour, corresponding to two notes outlined in the same colour on the body of the flute: fill each column to the upper notes for one key, and to the lower notes for another.
There are simple (and waterproof) song sheets included, but what I like best is the experimental nature of the toy. This is pure science — does the pitch go down or up if I add more water? — married to bathtub fun and to the delight of making music. And remember, music sounds great in the bathroom.
Go ahead and sing along, Dad.