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.

­In this activity, students self-organize according to periodic trends. I print out the attached cards onto card stock (each page will contain two) and hand them out to the students (one to each). Generally, we go outside and I shout out periodic trends (i.e. size, polarizability, ionization energy, Z_eff etc.) and the students run to get in line in the correct order.  I have a bell which I ding if correct, and a buzzer that I sound if incorrect. If incorrect, they have to try again.