Recently finished a self-balancing skateboard contraption, and thought you folks might be interested. Its more intro micro-controller and heavier on the machining side, but, it was an interesting experience on iterative design.
I'm really really jealous about all the nice toys you get to work with and what a great place where 'scrap' of this quality is available. Tinkerers paradise :)
http://www.toysrus.com/product/index.jsp?productId=42682786&...
It was really hard on your shins!
My favorite part was that the drive train in that generation meshed directly into the knobs on tires.
I have two nephews that would flip out with joy if they got one of these.
But what's plan b when something goes wrong? On the electric skateboards you can at least break in the good old fashion way.