When I was 11, I made Pinball machines. You would use Popsicle Sticks as the flippers, and I proper Clipboards up on books. Then I designed my pinball machines, with Plastacine, paper, and lego. I charged a nickel to my friends to play them.
Here's where, in retrospect, I think I was particularly smart. I also had a 'Comic Book Rental' program. It was more like a pass, and for one dollar they could take out 3 comics at a time, any time - and when they came over they could play the pinball for free.
I had to do it in my bedroom however, and only to friends. This kid had a storefront location! And access to all the cardboard boxes he could ever dream of!