Green Planet has a great list, we’re fans of number 1 and number 2 (the latter is listed below):

Laughter really may be the best medicine. Not only does it calm the nerves, improve the immune system, and break the ice, but it also burns calories. Yes, a good giggle provides a natural cardiovascular workout for your insides, by increasing your heart rate and circulation, and toning your abdominal muscles. Dr. William Fry of Stanford University says, “Laughing heartily five times a day has the same beneficial effects as ten minutes on a rowing machine.” What’s more? 10-15 minutes of solid laughter can burn 50 calories which can translate to 4.4 pounds in a year—hey, every little bit counts! – from Green Planet