peanut butter is a good one...you can also soak the kibble in water until it's soft and then stuff it in and freeze it, cream cheese, cottage cheese (i like to mix the kibble in with this stuff to sort of bulk it out) plain or vanilla yogurt, squeeze cheese, canned dog food, anything squishy that is safe food for a dog can go in...another good thing is to take a chewy, like a bully stick, cut it down so that it is just longer than the kong and shove it in the gooey stuff before you freeze it, or just shorter than the kong and shove it in w/o freezing it. the kong website has recipes too