Cardboard boxes with a utility blade or x-acto knife are really pleasant to work with. I've built plenty of things both for and with my son (now that he's a little older) for school costumes and projects. When we've got larger boxes, they're great for making a truck or a car to play-in. The durability of the box is usually pretty well correlated with the how long they remain interested in playing with the creation. Our latest creation was a cardboard knight costume with helmet, sword and shield. One of the more-freeing aspects of it all is that mistakes aren't a big deal because you've likely got plenty of replacement cardboard to try again.