If you can think it, you can build it

I found this cute site, perfect for anyone with a printer, paper, color ink and an imagination. They are FREE paper toys that you can print directly from the site: Readymech and construct with your child.























I like their modern, japanese look. However if that's not your thing I found a comparable site with a more tradition look, same concept:
The Toy Maker
(see below)

Bowling with Bunnies (pretty self explanatory)


















Marble Mice: place a marble under the paper mouse and watch them race on the table.