Buoyancy- foil boat. I really liked the idea on this site of using tupperware containers with pennies in them. I would extend it and see how many pennies make it sink, then put one less in so it barely floats. Weigh the container with the pennies. Take out the pennies, fill the container with water, then weigh it. It should weigh the same! I might have the science club get in teams and do some containers the same size, some bigger, and some smaller.
