Highly recommend supporting the Goldie Blox movement. I supported them on Kickstarter last year and recently received the first kit which teaches belt drives, force, friction, and wheels & axels. It’s geared toward girls in that it combines both reading and building. With that said, it is perfectly good for boys as well. If Legos and K’nex can be for girls, than Goldie Blox could be enjoyed by boys equally as well.
According to their website this is but the first of many kits that will teach young folks alot about mechanical and eventually basic electrical principles. Dare we see a chemistry kit too?