Hot wheels and toy trains.
Don't have a lot, but it's all my sons fault. At least that's the excuse.
I've taken him to car shows and swap meets for years. Pawed through endless piles of hotwheels with him for his collection.
Then the lightbulb went off. There's no way I can ever afford an XKE, but for about a buck, I can have a nice XKE hotwheel. OK! And there it started. I've become quite willing to buy a hotwheel of some car I covet.
And then there's toy trains. I always loved my O-gauge Lionels. Now I have a rational excuse to play with them. I'm teaching him about electronics! Ok, so he prefers HO, fine, I can't see HO very well any more (it shrank, obviously, and it got darker recently under them).
I've tried to get the kid to build models, but he's never gone for it. But, at the last train show he realized there are kits for train cars, and he was most interested.