Butchers puppy food and kibble mixed and packed in really tight. Plain natural yoghurt (or sometimes Vanilla) and fruit. Beef jerky and rice (boiled in chicken stock). Peanut butter and Primula Squeezy Cheese.
All the above combnations are made up the night before and placed in the freezer. My two are very experienced Kong lickers and need to have frozen Kongs as is slows them down (slightly).
Do a google for it, there are whole sites on Kong Recipes