I really like the Busy Buddy.
BUSY BUDDY dog TOY TWIST N TREAT small twist and treat | eBay
Regular small kibble works great. You can adjust the holes. You can shut it all the way but they can smell it and will work it open - takes more time. They flip and roll around, really keeps their interest. You can unscrew it all the way and pop it in the dishwasher. HUGE improvement over things like a Buster Cube. These are TOUGH. My Border Collie steals it and has not done any damage in a year. Soft so they don't hurt their teeth. All in all, a GREAT toy