This is a very simple boat your kid (or even you) can play with in a pool or a bathtub.
Tape a rubber band to the shaft, wind it up, put the boat on the water and release it.
You can print two boats to organize a race : the winner is the boat which goes the furthest !
https://www.youtube.com/watch?v=egY8GBtH8CU&feature=youtu.be
Also, if you're interrested, join my discord server to discuss about 3d printing, arduino, processing, ... and find many interresting electronic project ideas : https://discord.me/electronic_projects