OK, there are so many newbies here lately, so here is how it goes:
you can put basically ANYTHING in a kong, that is not harmful obviously. i order to prolong the time the pup is playing with the kong, you can stuff it with layers and freeze is. They absolutely love it, and teething pups get the frozen cool feeling as a bonus.
Options are: peanut butter, cream cheese, any other cheese, yogurt, vegetables (broccoli, carrots, butternut squash, etc.), kibble, treats, whatever!!!
There are recipes in the kong website and other sites you can find through Google. Here are some I found:
FunTimesGuide
Kong - look under tips and advice
Kong Recipes