I do like to alternate treats, but one of my (and my dog's) favorites is Buffalo Bites. It's like a buffalo jerky that can easily be broken into any size pieces.For a really high value reward I recommend using Ceasar's dog food (comes in the little individual containers). I put a pack into a squeeze tube and give a squirt as a reward.
High value treats for Rookie are little tiny bits of boiled chicken, cheese, and dried liver. If I'm going to be able to wash my hands easily and I want a REALLY high value treat, I'll use canned sardines or hot dogs.
I give my pup cheese or beef/chicken wiener chopped into little cubes. Since my vet suggested giving more fiber to my pup, I've been giving him apples, also chopped into little cubes, as treats. Don't give the core/seeds tho, it's toxic to them.
We don't really buy a lot of treats. When Tucker was a puppy, we used his regular kibble as rewards for good behavior. We also used plain Cheerios and very small pieces of hotdog. When we do buy treats, it's usually something like Bill-Jac liver treats.
I use a mix of regular kibble from her daily allotment, cheddar cheese bits, and whatever leftover meat we have! That way, even the boring old kibble has some extra flavor, and when she has an extra great response, I can treat with her favorite - real meat!