My most common combination is I first stuff it 2/3 of the way full with either kibble or frozen green beans, then add a spoonful or either cottage chesse or yogurt, then seal up the big hole with a little peanut butter and freeze.
Usually just dog food: Canned, canned and dry mixed together, or dry softened with water and mushed up. Sometimes frozen, sometimes refrigerated, sometimes just room temperature. Occasionally I'll mix in a spoonful of yogurt, banana, or peanut butter. I think someone here on DF said they stick a short bully stick in whatever filling they use and freeze it, I think that's an awesome idea but I haven't tried it yet.
We use unsweetened applesauce and plain nonfat yogurt as the "base" and add treats in there - peanut butter, berries, kibble, freeze-dried liver, leftover cheese and hot dogs from dog school...whatever's around.
I found some unsweetened applesauce with berries in it at Whole Foods. I was also thinking about picking up some liverwurst. Cream cheese is also a nice little treat.
In the kong I usually fill with a little peanut butter, maybe some liverwurst, and mostly goat yogurt and freeze. I use goat yogurt only because its a little easier to digest than the cow products - but its expensive so I think regular low fat, plain dairy yogurt is fine its just a personal preference for me to use goat products. I have used some cottage cheese in the past as well.
As sassafras mentioned - I also stuff a bully stick in there too before freezing - makes for an entertaining treat!
I have also this rubber toy which can be stuffed with treats - I stuff with some mother nature milk biscuits, frozen green beans, and some cubes of goat cheese. Since I am gone during the work week for 10 hours during the day, I stuff both the kong and the treat dispenser for Thumper, as well as he gets his antler too, and leftover marrow bones. I do what I can to keep him occupied during the long days and he cleans em all out except he can't get the cookies out of the treat dispenser.. but has no probs getting the beans and cheese lol.
Cash on the other hand, well he's funny. He wont chew or anything while I'm gone - If i give him a bully stick or a marrow bone he stuffs it in the couch and leaves it there until Thumper, or one of us finds it. He only chews when I am home, so he usually gets nothing during the weekdays but he doesn't really need the entertainment as much as the pup for 10 solid hours.
Buster is only 3 months old and still needs 3 meals a day so I stuff his with chicken pieces and seal it with plain, low fat yogurt. For Lola's I smear peanut butter around the inside and then add a piece of cheese, a frozen berry or 2, and some dog treats then top with the yogurt.