Another session I developed for the Maker Club for kids at my work (a public library).

It's dead simple, but the kids absolutely loved it.

We divided them into teams and gave them access to a bunch of supplies and recycled materials with the goal of creating a boat or craft that could float a potato. The team whose boat could hold the most for the longest without sinking won.

Step 1: Supplies

Collect and clean a bunch of recycled containers and materials. The more that are plastic the better, but sturdy cardboard will also work (though of course a boat made of cardboard won't survive multiple floating/sailing attempts). Styrofoam is also great. Be sure to have at least a few containers with lids as they are great for adding buoyancy.
Hot glue guns
Various types of tape (duct or packing is best)
Elastics
Scissors
A kiddy pool or large plastic container full of water
Several large potatoes
Towels for drying
Prizes for the winning team/group (optional)

Step 2: Talk About Buoyancy

I gave this intro talk to the kids:

“We have here lots of different materials you can use, and lots of glue gun and tape. You will have to use your knowledge of science from school, and what I’m going to tell you in a second, to make a boat that will be able to float a potato.

So, now a little bit about why things float or sink:

Imagine yourself floating in a pool. As you float, your weight presses down into the water; the water presses back, pushing you up. When you get into the pool, your body displaces a volume of water (the "hole" in the water that your body fits into). As long as the water your body displaces weighs more than you do, you float. This is why a big piece of solid metal would just sink into the ocean, but a ship made out of the same amount of metal will float. The metal is hammered flat and is hollow. Because the boat is so large it displaces a lot of water. This water weighs more than the metal because the metal (the boat) is hollow. You can also think of it this way, when you are in a pool you also weigh less than the water you’re displacing because your lungs are full of air, like a balloon, and like a balloon, the air in your lungs lifts you to the surface.”

Step 3: Let 'em at It!

We had one station per group set up with a glue gun, tape roll, and scissors. After half an hour we made everyone stop and the floating contest began!