We go through a lot cardboard in ArtBreak. It is used for constructions, collage background, handmade book covers, dinosaur feet, robot arms, paintings, drawings. So I collect cardboard boxes and used tri-fold project board and prep it for ArtBreak using a mat knife and a long metal straightedge.
Voila. It is very satisfying to create a beautiful stack of cardboard that will be enticing to students.
Here is a good tutorial on collecting and working with cardboard. It gives technical tips useful for ArtBreak rather than “projects” for children to copy: http://www.ikatbag.com/2011/03/how-to-work-with-cardboard.html