Last summer my littlest son and I made walnut boats.
He just loved picking out the colors and painting!
Once the shells are really dry, take some modeling clay and stick it into the shell.
We then made little sails out of toothpicks and colorful paper. This was one of his favorite parts too. To make the sails, fold the paper in half and draw a triangle - one edge on the fold. Cut the two edges not on the fold. Open it up, spread on glue with a gluestick and stick the toothpick in the crease. Fold together and there is your sail.
Stick it into the clay and you have a cute little boat.