Simply take a large dish, and fill it with liquid mercury. Float things on the mercury. Rocks, iron nails, witches, lead shot, you name it. It's best to start with the least ridiculously dense things, and build up to lead shot.
WARNING: Mercury is way bad for you, kids. Use appropriate caution.
OTHER WARNING:The mercury will amalgamate with many of these things (like lead), so that this sample of mercury will be contaminated with other metals, so this sample of mercury should be dedicated to this demo, and can be reused year after year. It should be clearly labeled with appropriate warnings about other contaminants (e.g. lead).
Large recrystalization dish
If they ooh and aah, you did it right. A good demo for early in genchem, perhaps when discussing density.