What it shows
We have three 20 oz. soda bottles, one filled with water, one filled with sand, and one filled with air. A spring scale shows the water-filled bottle to weigh approximately 6N in air, and nearly 0N when it is fully submerged in a large container of water. Since gravity is still acting on the bottle when it is submerged in the water, there must be a force of 6N pushing up on it. This is the buoyant force.
We can do the same experiment with the bottle of sand. This bottle weighs roughly 13N in air, but when it is fully submerged in water it weighs 6N less. Even though the bottle of sand has more mass than the bottle of water, they both experience the same buoyant force because they both displace the same volume of water.
As an added test, we can use a string and a horizontal bar attached to the bottom of our large container to show that we need roughly 6N of pulling force to fully submerge an empty 20 oz. bottle.
Setting it up
Use the specially modified Belmont Springs water jug and a 20N spring scale. This demo is guaranteed to make a mess, so use a plastic cart and have plenty of towels handy. Make sure none of the bottles are leaky!
Measuring the buoyant force with the empty bottle can be tricky. There should be enough water in the jug for the bottle to be completely submerged without the string or the hooks snagging on the bottom bar. To avoid awkward minutes of splashing around in front of students, take time before lecture to make sure the string is looped under the bar, and that the ends are conveniently draped over the sides of the jug for easy access. Pull straight up with the spring scale with just enough force to hold the bottle in equilibrium, or else you may get inconsistent results.