Crazy Cartons was a great idea from Post cereal. You take the carton that you normally serve milk in, fill it with breakfast cereal and cover it with cartoons. It was supremely kid friendly and was the prefect kid treat. The cartons were so respected in my house when I was growing up that well after they stopped making them I would find the cartons folded up in drawers, left there by my sisters years earlier.
Here is a promo they did for a Crazy Carton contest that challenged kids to come up with fun ways to use their crazy cartons. I like the kids train idea, but I think if I were suggesting something it would be a robot…ooh or a rocket…or a robot rocket!