I mean, you can pretty much stuff anything in there, lol.
I buy plain non fat yogurt, fill it up inside the Kong, and then put in the freezer. He looovesss frozen yogurt. Sometimes I'll also put little pieces of his Zukes treats in there so it gets frozen in the yogurt.
A lot of folks also put frozen peanut butter in the Kong, but Jackson doesn't like peanut butter so we don't do this, lol.
I dog-sit for a Cocker Spaniel, and he gets these peanut butter biscuits that fit right in there... and he works for a while trying to get it out.