housing and recreation for guinea pig

Question
Can you recommend a material, possibly something recycled, to allow our guinea pig to get out for a little exercise?  He is our class pet in a preschool 3-4 year olds.  The children are very good with him, and it would be extra fun to involve them in this project.  Any knowledge of materials to stay away from for health reasons?  Thanks for your input!  

Answer
First, please make sure his cage is big enough for him, and see if it is possible to get him a same sex friend.
www.cavycages.com
These cages are great and you can use extra grids to make a collapsable playpen. Throwing down a plastic shower curtain from the dollar store is what most people use to cover the floor.