Potato plus anything else mixed in makes a great paste to put into Kongs - frozen or unfrozen. Also, soaking some kibble and mushing it in there is a good option. I have had so many Kongs through the house over the years that it is rediculous. One of my dogs has discovered the best way to get the yummy stuff out of the Kong is to just eat the whole thing - he chews open the Kong, eats all the good stuff and, depending on how hungry he is, will either eat the rest of the Kong or leave it it pieces all over the floor. He has even managed to do it with the Xtreme Kongs!