For his birthday last year (or maybe for Christmas), Son got a K-Nex R/C car and a K-Nex roller coaster. The car was a bit over his head to put together, and a bit hard to control, and so we shelved the roller coaster, and sort of forgot about it. Well, the recent amusement park trips got him remembering it, and we pulled it out and put it together.
It was still a bit over his head, as far as doing it himself, so he clicked the parts together, and I told him what to click. It went together over a few nights of effort, and is actually a really cool toy/model. As a roller coaster, it's small, but works quite nicely, as the single car grips the rubber tubes with wheeled grips, and just whips around the track. I mentioned it being small; it's a little figure eight, with the track going under the lift and back around to the bottom.
Of course, now I want to get 5 or 6 more such kits, and put a real roller coaster together in the basement. :)