Take a look at Ambrose Spencer's SunToys at
http://energyvision.blogspot.com/...
and you will see why I say that if you are going to do only one public solar demo, you should do a solar fountain.
A floating solar fountain can be bought for $60 or less. It will be less impressive than Ambrose's larger model but add a basin and some water and you still have a great display that kids love to turn on and off with their shadows.
It will naturally become a solar wishing well.
Floating and other solar fountains available from
http://www.kineticfountains.com/...
http://www.siliconsolar.com/...
http://www.improvementscatalog.com/...
and
eBay
Buy one and set it up at the local farmers market or public square. Raise a little consciousness and make a few rainbows.