My son is 4 and a half and loves playing with Legos. He's just beginning to start building small ones on his own (Lego City police motorcycles and the smaller cars), but he really enjoys playing once they're assembled. Thus, we have a lot of Lego City police and fire vehicles.
I've wanted the large VW Camper Van for a while. It just looks fun, and I missed out on Legos as a youngster. In fact, I can safely say I never had Legos until we purchased the first blocks for my son. This is quite the appropriate father/son gift, given his current interest, than this puppy. We started on it yesterday and will chug away some more this evening when I get home from work.
My wife had other things in mind as well. Floor mats for the ST, various tools, etc, but this is something my son and I can spend a few hours at the table putting together and enjoying once it's completed.