Aquinas, thanks for the links to those books! They look awesome and have gone on his wishlist.
I think someone on here had suggested 'Car Science' by Richard Hammond and we've really been enjoying that book although I'm learning more from it than he is so far (it's geared a bit older).
daytripper, we have one of the Battat take apart cars and he does like it. Another big hit (for others who have similar kids) are the mini Automoblox and Lego Juniors sets.
I'll see if I can get to a thrift store and I'll let everyone know if we hit on a super great project. I'm also thinking that some of the stuff we get in happy meal toys might work well.
