There's a brand of treats called "Northern Biscuits" and they fit PERFECTLY snug into the kong. I usually put in a couple of the zukes super green's treats, then break a biscuit in half and put one half in each kong to trap the smaller treats in until they can get the biscuit out. Keeps the girls occupied for about 5-10 minutes and makes cleaning the kong A LOT easier (I dont have a dishwasher...).
Lady (1.5 year old tricolour) & Gracie (4 year old blenheim)
"Happiness is a warm puppy" - Charles M. Schulz