We tried Wolf Elective 5F tonight. Building a rubberband paddle boat. This was an easy elective and the boys adored it. We chose to make our boats out of high density foam flooring and coroplast board (glued on top) for strength. We cut the foam boat out and then laid a coroplast layer on top to add rigidity. Odd I know. but that's just what we happened to have handy.
Here's something interesting. If you choose to build this project, experiment with the size of the paddle. The shorter ones go faster, but the longer ones are stronger. Interesting experiment if you want to teach your child a few physics lessons.