Meant to post this weeks ago... I think I may have found that late-summer project for fun that I can do even with a nagging back, and man-o-man, is it every nifty:

It's called a Hydrofoam. Still pictures don't do it justice, so take a few minutes and watch this movie. Be sure to keep watching, it's starts out kinda cool and then quickly gets to the point of awesome. You'll see what I mean...

Thanks, Dave for the link!

There are additional forum threads around with plans and whatnot as well, for those interested in building something themselves.