I just got the I-Qube, which is a cloth cube that four squeaky balls fit in. Buster LOVES it, and it's his new favorite game. Bailey takes no interest in it, but it has been a good addition to the toy collection for Buster. Bailey's favorite are toy ropes. They also like Busy Buddy items, we got those at a local animal supplies store. Oh - and plastic water/pop bottles (under supervision) & also ice cubes. I got the I-Qube at Petguys.com (among other things like the hide-a-squirrel, but they don't really like that one) - check their Interactive toys out - they have lower prices than almost anywhere I've seen & also have good discounts on kong fillers in the treats section.
IQube:
Kyjen IQube Puzzle Plush Dog Toy Junior. Happy shopping!