Uberstix UberDragon 71 Piece Set
The UberDragon Kit is part of the "Animal Kingdom Series" and are a perfect example of why the UBERSTIX build system is so special.  The frog leaps, the crab's legs move, the dragon slithers, and the bird and bat's wings flap.  The animals are much more than stationary models.  As cool as this sounds, it also teaches youngsters about the workings of movable joints, and when it's approprate to engineer a creation with movable, as opposed to non-movable, connections.  Kids can build a host of other models with these kits as well, including a MiniPult that launches small objects.  As a bonus, the kits contain a one-hour simple-machines lesson plan.