Make a popsicle stick paddle boat that’s powered with a rubber band. This video from DaveHax demonstrates how cutting and gluing together some lollipop or popsicle sticks can make a simple paddle boat structure. Sand them with sanding paper if you want it to have an extra-fine finish.
Add an elastic rubber band and keep twisting and twisting its attached paddle to see the boat take off in the water.
There’s more DIY science and craft videos on this site, including soap-powered boats, how to make a Popsicle Stick Chain Reaction, and how to make Leonardo da Vinci’s self-supporting bridge.