I've tried putting dog cookies in kongs, cheese bits, and making kong peanut-butter popsicles, but zoey just doesn't like them. She has trouble getting the treats out, so she gets bored quickly and just gives up - she is such a quitter...
But zoey LOVES her UFO thing (the same one jen posted). We can open and close it to the right size, and it is challenging for her, but not so challenging that is frustrates her. We'll put in cookie bits, but her very very favorite is when we leftover steak (I put them one or two little chunks at a time and refill it when she gets them out).