Instructables - a community DIY site with many projects
Some of the projects are involved and definitely adult/techie oriented, but others require only simple materials. And there are a host of variations on the Mousetrap car:
http://www.instructables.com/id/Mousetrap-Powered-Car
http://www.instructables.com/id/Mousetrap-Car-Racer/
http://www.instructables.com/id/Lego-Mousetrap-Car/