Falling in an evacuated tube at the same rate.
What it shows:
In the absence of air resistance all bodies, regardless of size or weight, fall with the same acceleration at the same point above the Earth. Here a feather and a dime (see Comments) fall under the influence of gravity in an environment where there is no air to mess things up.
How it works:
The fall takes place within a sealed clear plexiglass tube 1.5m long and 5cm diameter (figure 1.). The valve at the base of the tube screws into a vacuum pump platform to stand upright. Evacuate the tube using the vacuum pump (which takes about a minute) and close the tube's valve to isolate it. The tube can then be unscrewed from the platform and tipped end-over-end by the lecturer.
Setting it up:
Place the platform on a cart or the lecture bench with the vacuum pump beside it. Leave both the tube and vacuum pump valves open. It is good to show the fall with air in the tube first, showing the effect of air resistance on the relative fall rates of the feather and dime.
Instead of a feather, we use a folded piece of brightly colored tape, and for the dime, we use a quarter. This is for visibility reasons, plus the tape is less susceptible to static charge on the tube. The fall duration is close even with the air, so it is a demo to repeat several times.
The video below shows the free-fall acceleration of a golf ball compared to that of a paper cupcake liner. Both objects fall inside a plexiglass tube. The first trial is done with the inside of the tube at atmospheric pressure. Air from the tube is then pumped out, and we see the objects fall again with a different result. (Produced for the Harvard Smithsonian Center for Astrophysics by Alex Griswold and Clive Grainger.)
A video of the hammer and feather drop on the Moon by Commander David Scott of Apollo 15 would be good accompaniment, as would an account of Galileo's (supposed) demonstration of dropping two canon balls from the top of the tower at Pisa.