Finding the center of mass with your fingers
To demonstrate an easy way to find the center of mass of some objects.
Take an ordinary meter stick and place one finger from each hand under it to support it. Move you fingers toward the center of the meter stick. Once one of your fingers passes the center of mass the stick should fall off of your fingers. Try to stabilize the stick and the location at which your two fingers touch while the stick is still supported is the center of mass. To prove this you should repeat this experiment after hanging a weight off of one end of the stick.
This demo can easily be done by many members of the class at one time, the students can use hats and other objects they might have to add weight to one side to avoid having to bring that many weights.