I got Stewie a Mazey Munch Ball (4″ diameter) but he hasn’t really shown much interest yet. I, however, think this toy rocks. The best treats to put inside are small balls like NutriBerries.
You’re supposed to swap out toys on a weekly basis so that the parrot doesn’t get bored. As far as I can tell, Stewie doesn’t seem to play with any of the toys he has in his cage, except for his mirror. So I need to do some more experimenting to see what he likes (and/or teach him how to play with the toys).