I've been mixing peanut butter and plain yogurt together. A dab of PB at the small end of the kong to plug the hole then i add the yogurt and pb mixture until almost full. I then FREEZE it and when Zoe gets it before I leave, i fill it with kibble and biscuits and liver treats! She absolutely loves it and can't wait for me to leave! lol Too bad it won't work for Copper!
Freezing it is great because she is teething right now so it helps soothing her gums! She lost two teeth in one day!!
Never mind the dogs, beware of the owner!!!