RC cars, it's a no-brainer. The hobby-grade RC cars have much of the same functional suspension & steering parts as full-size cars, but with electronic motors, speed controllers and servos instead. They can get spendy too, but decidedly more affordable than full-size cars. And you can choose from a plethora of makes, models & bodies. You can go for drag cars, offroad cars or on-road cars. They look great on your shelf. If you like wrenching on stuff, RC cars are a great hobby. The only downside is our failing eyesight...makes peering at tiny screws and parts a bit more challenging.
The hobby seems to be declining though as most kids nowadays want to buy something that's ready-to-run out-of-the-box, building and wrenching seems to be dying out with today's "I want it NOW" generation.