Voodoo mentioned to me about freezing kongs and this has been great advice they last much longer! Peanut butter is good but you could try cream cheese or the puppy paste made especially for kongs. What we do is block the end with some paste (the small end) then fill it with kibbles, treats, meat etc then block up the other end with paste as well they have to work to get the treats in the middle out. If you look on this website theres lots of good ideas for recipies and also a good technique for stuffing them!

Kong Stuffing Recipes

Sorry if the link hasnt worked I've never tried to put one in before!