Dancing raisins, mothballs, and other variations
Basic mothball and raisin theory
A truly silly trick if you happen to have extra mothballs. Mostly fill a glass jar with water. Add a little vinegar - 1/4 to 1/3 cup (60 ml) - and 2 tsp. (10 ml) of baking soda. Stir gently. Add a few mothballs. As long as their surfaces stay fairly rough, they should begin to bounce up and down. This will also work quite well using clear soda water with the mothballs.
Raisins and alka seltzer or clear soda will perform the same way. The irregular surfaces on the mothballs or raisins hold some carbon dioxide bubbles. When enough bubbles accumulate to lift the weight of the mothball (or raisin), it rises to the surface. There, some of the bubbles of air escape into the atmosphere, and the mothball/raisin, which is denser than the water or soda, sinks to the bottom to start the cycle over again. The effect will last longer if the container is sealed, as less carbon dioxide will be able to escape.
There are quite a few other small objects that will work. As I am writing this, in fact, I just received a note from someone about an oscillating couscous grain in seltzer water! (Thanks, by the way.) The key is that they are able to trap air bubbles on their surface, that they are light enough to be buoyed to the surface by the bubbles, and that they won't dissolve in the liquid.
Fun with champagne
If you happen to be drinking champagne or a sparkling wine, raisins will bounce in the glass for a very long time. Of course, very few of those who enjoy a glass of champagne will have the patience or willpower to not drink the experiment.
Even more fun with gelatin
Thomas Castagno writes to add that "if you pour (your favorite clear lemon-lime soda) into a clear glass cup, add some salt to the soda and put some Jell-O into the cup that after a while the Jell-O will bounce up and down inside of the cup as air bubbles accumulate and come off of the Jell-O." I haven't personally attempted this yet, but I imagine small bits of (lime green?) Jell-O are used.