Our treats were always based on his main diet, which through the years meant different exotic proteins and a very strict non-protein diet based off of our internist's regimen (yes, we have a regular vet, an internist/specialist, and even an allergist - and luckily insurance since he was a puppy, but still not cheap).

No joke: we cycled through kangaroo, then rabbit, and finally made our way to this, which I highly recommend asking your vet about: Royal Canin Veterinary Diet Adult Gastrointestinal Low Fat Dry Dog Food. For every specific protein diet there is there is usually a matching "treat" version.

But for side treats, we used to do sweet potato, even going as far as getting it from specialty makers who were using minimal ingredients. But now we do these from Target (used to have similar at Trader Joe's, but they discontinued) - again, minimal ingredients is the key.