I like to use Peanut Butter, and freeze, or get the cheese in a can and put that in there and freeze. For young pups that are teething I will close the small end up with peanut butter fill halfway with water, and then seal with peanut butter again. They all seem to love this.
Incidentally, never leave a GSD alone with that stuffed Kong. I was watching TV and heard a loud POP!! sound. I looked over, and my GSD had bitten the Kong into two halves! The Kong was the name-brand, not a knock-off, and it was not faulty nor splitting along a seam... it split with an upper and lower section seperating. My dog was 14 at the time-- a senior!
Geez Patti, what were you feeding your senior super strength food? LOL
You could try using frozen chicken broth as well. Patch up the small hole end, and fill with the broth. Put it in the freezer over night (or until frozen) and then give to the dog. They will just lick it over and over until they get to tired. Its like a chicken lolipop! We do this with icecubes too. Freeze a ice tray with chicken broth in it to give to the dogs as treats on occasion.
I like to stuff peanut butter slathered dog biscuits into the Kong sideways...(the peanut butter acts as a mild glue). You know when the peanut butter is about gone...when Pal starts dropping the Kong on the floor to jar the biscuits loose. He's got it all figured out, just takes it awhile. Any other ways I can try to make it more frustrating for him?