Tennis balls

The guinea pig will chew on just about anything. I've had tennis balls in my cage before and they never chewed on the fuzz. They mostly left them alone though. Plastic cat balls are good as long as they are well constructed and can't be broken, or the bell can't come out. Just make sure the cavy can't ingest any part of it.

Superpet make balls for ferrets, they are very much like cat ones but alot stonger!
They are called ferret shake, rattle and roll balls.