This is one of my faves. Get a copper plumbing pipe at your local hardware store. A 3-4' section outta do just fine. The diamater - either 1/2 or 5/8" - depends on the diameter of the magnet(s) used. Also, obtain a "cow" magnet or cylindrical alnico's. These are easily obtained thru toy stores and/or your neighborhood scientific house. While holding the tube vertically, hold one magnet inside the top of the pipe and one outside. Drop them simultaneously and at the same time. WOW! The one dropped OUTSIDE the pipe falls to the ground (hopefully protected by foam or a box or something...) first. The magnet that fell through the pipe took its good ole time! WHY? A moving magnet will set up an electrical current in a conductor, called an induced current. The falling magnet induces an electrical current inside the pipe. However, this induced current will itself create a magnetic field, since all current-carrying conductors have a magnetic field around them. The trick is this, the induced current creates a magnetic field OPPOSITE that of the mahgnetic that created it. Thus, the two magnetic fields fight each other and the magnet is actually repelled UPWARD with a force that will decrease the net force acting on it.This works quite well with a bunch of small alnico disc magnets. Their strength induces a very strong current and therefore a strong opposite mag field. To enhance this effect, you can obtain a sheet of magnetic "viewing" paper from your friendly science house. Wrap this paper around the pipe and it changes color as the magnet drops past it. You can actually SEE the magnet fall from the outside. COOL!
If you have a disc alnico magnet that is very close to the diameter of the pipe, watch it thru the pipe as it falls. It actually rotates, changing the direction of the induced current as it falls. This actually sets us a type of AC current in the pipe. Cooler!