I had read somewhere (not sure where though) that the glues they use to hold them together are not bird "friendly." I currently give him kraft paper (I have a whole roll of it) that I twist and weave through the bars. I also lay it on the floor when we are foraging as a pack. It's pretty crazy to see myself, 3 dogs, 4 cats and of course Gizmo searching for tidbits under the paper. I can only imagine what a neighbor would say if they saw us!
His favorite things are gift wrap tubes because they can go from one side of his cage to the other. He gets great delight out of making a hole int he center so that the roll falls to the bottom for more pouncing action. I could talk about him all day. I will look for the tube info and post it if possible.