new PhET Buoyancy sim.
Pool Cubes 2: Buoyancy (PhET page)
The purpose is to investigate the nature of the buoyant force and to see the role it plays in determining whether or not an object floats. The ability to use a variety of objects in the liquid and to vary the density of the liquid makes a number of scenarios possible. Real labs are great, but do you really want to deal with a broad variety of liquids (oil, gasoline, etc.) in the laboratory?
I wrote the activity for my AP Physics B students, so it gets mathematically robust toward the end. Nothing they can't be able to handle with a bit of algebra.
The groovy "3-D" graphics and textures used in the new sim are great. The science has to be right, of course. But once the substance is established, why not have some style?