My kids are always playing their own version of the game "Sink or Float". We fill up a plastic bin with water in the backyard and they run around looking for things to test. For example: buttons, postcards, leaves, rubber bands, small rocks, popsicle sticks, keys, bouncy balls, pez dispensers, etc.
To make your own juice box boat, follow these easy steps. 1. Cover an empty juice box's straw hole with tape. 2. Cut a 4" high by 3" wide triangular sail from a Tyvek envelope (I used an old USPS Priority Mail envelope). Decorate the sail with colored tape and make a little flag. 3. Push a small wooden skewer through both sides of the box so that it sticks out underneath about an inch or so. 4. Use tape to attach your sail to the skewer.
We've made several versions of these boats this summer. We take them down to the little creek that runs through the regional park nearby and see how far they can sail without tipping over. (If your boat seems especially tippy, cut a narrow slit in the top and slide in a few pennies.) Happy sailing!