The hide-a-treats are deffinately the best for stimulating the dog's mind without them running around.
Toys like the kong stuff-a-ball, kong jumping-jack, and even classic kong can be filled with all sorts of treats that they have to work to get out - and they even have some recipes on the kong website (
http://www.kongcompany.com/worlds_best.asp) for things to fill the classic kong with. Nylabone, booda, and planet dog also make treat dispensing toys. There is also a brand that makes purple stuff-a-treats - I can't remember which one it is right now, but it's actually zoey's favorite...
Another thing to do to take it a step beyond just getting out the treat is to play hide and seek with it. You can put a treat in the toy, have the dog smell it so they know what it smells like, and then have the dog go outside or in another room and hide the toy. That way they get the challenge of finding the toy, and the challenge of getting the treat out of the toy