A bumpy toy small enough for an adult cat to pick up and carry around, or bat around on the floor.
Thick enough that even stronger-mouthed cats shouldn't be able chew through it.
Some smaller dogs may find this toy enjoyable as well (WARNING: May be dangerous for larger dogs, who may be strong enough to break it, and may even try to swallow it whole).
NOTE: This may be a too large for smaller kittens to pick up, but it should be light enough for them to bat around (6" diameter).
NOTE: This has only been tested by 2 cats so far. One loves chewing on it and carrying it around. The other finds it less exciting, but does play with it some (but he tends to be the less playful one anyone).
WARNING: The corners of each bump can be a ltitle sharp. This does not bother my cats, but could be an issue for cats with sensitive gums.