Erector sets...

I had a huge erector set when I was a kid,along with about half a ton of legos and knex kits,at one Point I built a walking robot with a Gatling gun,rocket launchers,working hands,etc for a school project. That stuff really makes you think and IMHO is a great teaching tool. Honestly if I see a kid that excels with erectors and stuff then they are probably pretty smart and can comprehend some pretty advanced stuff. Nothing like working with your hands at a young age to stimulate a young mind.