Here's what I'm thinking for my builder boy: Erector sets are good, or so I've been told. We'll get one and see how he likes it. My brilliant (or possibly ridiculously bad, we'll see) was a box of supplies to make his own robots. There are bunches of robotics instructional sites, and I think I might get a bunch of pieces that are used in those, then show him one of the sites and let him pick the one he wants.