Reward: Shake Up Silver!
Building boats and testing their buoyancy is a wonderful scientific endeavor.
Start with a variety of craft supplies (e.g. popsicle sticks, craft foam, straws, jar lids, pool noodles, etc…) and try to make something that floats in the sink. You can also experiment with asking the question, «How many rocks will it take to sink the homemade boat?»