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.