I have been working on windup helicopter designs for a year: one that you make
the propeller out of a 2-liter bottle that goes much higher than tall trees (above);
and an easier kind for young kids. Find out more here.
I learned so much from the Science Online videos and I'm sad that the force behind them--Wayne Campbell--has passed. My favorites are the historical invention ones such as Faraday's first electric motor and Bell's photophone. Wayne did not just talk about them; he showed us how to make them! All of his videos are treasures.