When I was a boy, I remember making walnut boats like this with my Grandma. My sisters and I would play with them in the bathtub. They were so simple and fun. This morning we had some walnuts in the kitchen, so I made some boats with the kids!
Just melt some candle wax into a walnut shell. When the wax gets a little firm, stick a toothpick in it for the mast of the boat. Let the wax cool. (We stuck ours in the fridge for a couple minutes.) Then attach a paper sail by poking it onto the toothpick. Ahoy!