I think I could start a catalogue of kong recipes

Almost everything I put in kong, I freeze to make it last longer and reduce stain damage to my carpet!
Mason likes scambled eggs, natural yogurt with fruit, gravy with chopped up ham, reduced sugar jam, peanut butter, mashed bananna, mashed potato, left over risotto etc etc
If you get a small piece of processed type cheese and use it as a 'bung' in the small hole you can fill it with water with bits of biscuits or meat and freeze it upright. Like an ice-lolly but harder to get at

I found this was great when it was hot to get him to drink more water (fussy little git)