Huge, black, plastic bag. Open one end and run across the playground to fill it air. Tie the open end and allow the balloon to sit in the sun, absorbing radiant energy. The air inside the solar balloon will expand and become less dense than the surrounding air. At some point in time the balloon starts to rise. Great way to teach radiation, convection, density, and also explain how a hot air balloon works.
LL Demos & Labs: Solar Hot Air Balloon