I used to have a cat who loved chewing the top of cardboard boxes, even the ones which were intended as her beds. She left little rows of teeth marks in them, almost like a teething ring though she was an adult cat.
Others just love sleeping in them. For kittens, you can turn them into great toys by cutting several kitten-sized holes in the shallower boxes and they chase each other in and out of them. I saw this on a youtube vid. Will try to find the link.
Hope this works:
http://www.youtube.com/watch?v=jL1gPsYmYpM
