We use kibble and maybe a couple of dry treats (like Fruitables) or chopped veggies (carrots, green beans) and usually seal it with peanut butter. Levi's gotten pretty adept at getting everything cleaned out of his Kong, so it doesn't last him nearly as long as it first did. During the summer, I freeze watered-down chicken broth, yogurt, etc. and let him chew on it on the deck outside.
He also really loves to play with his Kong when there is nothing in it at all.
