Gravity's effect on different masses.
To demonstrate that all objects fall at the same rate regardless of mass.
Ping pong ball
Steel ball, same approximate size as ping pong ball
An object to prevent the steel ball from harming the flooring
Take out the ping pong ball and the steel ball and ask the class which will hit the ground first. Then drop both of the balls at the same time from the same height. The balls should contact the ground at the same time.