My dog (Maltese X) loves his Kong. However, he's got a food allergy that we've been trying to diagnose, so buying the store-made Kong stuffing to give him a special treat just is a bad idea. So I got creative. I found things that he likes, that don't make his allergies worse, mixed them together and fussed with the consistency, until I got my home-made, healthy Kong-stuffer.
Here's what I do:
Take 2-3 dry dog cookies (or use kibble; vary depending on size of treat), crunch them up using your fingers, or squishing them with a rolling pin until you have some crumbs & some pieces that are smaller than bites but still in chunks. (If you use kibble, you still need to crunch them up because if they swell with the moisture they might not come out of the Kong).
Add 1-2tbsp peanut butter (smooth or crunchy).
Add 3-4tbsp plain yogurt (one at a time, mixing between each).
Mix in bowl until this is a soft, but not runny paste. Toothpaste-consistency seems to work best. If it's too soft, add more pb, if too thick, add more yogurt.
Stuff in Kong with spoon & give to your dog! I know mine loves me when I give him this.
I have the smallest Kong available so I always have left-overs of this mixture. I just put it in a tupper-ware container and keep it in the fridge. If it's too dry next time, add more yogurt!