cardboard boxes
Young cats especially like to chew on cardboard because it scratches their gums--when they're teething (at around 5-7 months) their gums are itchy and inflamed, and rubbing them seems to help. Some cats just pick up the habit and keep it forever! My old girls were styrofoam sculptors, too good for cardboard I guess!
And to Xander, buddy you need to learn not to play with hands and feet. Cute now, but when you're 15 pounds, you're gonna put your folks in the hospital with punctures! NEVER EVER play with kittens with bare skin!! It's a habit much harder to break than to make. I've know adult cats to get euthanized for biting when they were deliberately encouraged to do it as kittens.
Cheers,
Dr. Jean