Freeze it!
I do have a small kong and I've stuck Alpo soft-chewy treats in it and she chews for a couple minutes but then it usually falls out and she's done with it. I've been thinking about trying peanut butter.
Karen- The same thing happened with Diego. But someone suggested to me *freezing* whatever you use to fill the kong. It takes longer to get out and therefore they work harder. It's great! (It won't be long before it's time to get a bigger size) -Anything soft works really well. I use one or more of the following combinations: small training treats or kibble, peanut butter, honey, applesauce, carrots, green beans, canned dog food, mashed pumpkin/squash, apples, bananas, blueberries, oranges. During the warmer months, applesauce and/or juice is a big hit!
This of course is an outside-only activity :p
A word of caution though. It took me awhile to figure out that Diego has a sensitive tummy. Because you have a pup, and you know peanut butter works, start there. Then add one or two things and see if she likes it (and it likes her back!) I don't totally agree with your vet about the people food thing, HOWEVER I'm sure it has to do something with making sure the puppy gets well balanced nutrition and preventing a potential picky eater. - Lisa