Just pop to your nearest supermarket to see what size boxes they have that you can pick up for free.
This idea is from http://lifeandstyle.alexandalexa.com/cardboard-craft-projects-keep-entertained-christmas/
Im just loving this amazing car made from cardboard. Pop on over to http://www.crafthubs.com/mimi-flowerpower-cardboard-box-car/19626 to find out more.
Or how about this amazing cardboard box craft room? Find out more at http://www.thecraftycrow.net/2013/06/cardboard-box-art-room.html