I picked up some plywood dinosaur models at Harbor Freight for about a dollar each when my son was born (he's 4-1/2 now). We've built two together already and have a few more to do, He brings them to school for show and tell (they can't bring "toys" but I guess these are scientific) and he brags how he built it with the help of his dad. He spends about ten minutes and then I do the rest, but 4 year old boys don't have much of an attention span. I have to glue them back together and reinforce them when he breaks them, but Elmer's glue is cheap enough.
I've also bought a few of those snap/screw together airplane kits at a local air museum and he helps put those together and then plays with them. He has a Fokker Triplane and a Mustang. He loves going to the airport, primarily for the gift shop experience.
Don't tell anyone, but I let him sit on my lap and drive my truck too (not on the open road obviously- vacant parking lots)...

Hopefully we won't get a visit from child protective services anytime soon.
I have a go-kart to fix up for him in the next couple of years (I always wanted one as a kid), but it's hard to find a place to drive it in the city without getting into trouble.
My wife is constantly amazed at this problem solving skills and use of logic. Maybe it is just because it is her son, but I do think he has a bit of the Dilbert in him! No doubt he'll be a gear head like his dad, and he'll end up with a wife that complains about all his hobbies too.
