Kong has a couple of different things to stuff it with. I've never used the can stuff but I do use the other. It's like a stick that you stuff into the kong. It's hard and they chew on it. The pain in the butt thing about it is though, when you have to push it out! You have to find something to push it with. I have a small pair of scissors that I hubby uses to push it thru. Be care though, you want to watch for when the stick gets too small, like any other treat, they can choke on it. They also have small wafer like pieces that you can get too, they go thru these too, too fast so I stay with the stick things.
I heard that you can put a little peanut butter in it and freeze it. I've not done that, I just don't know if it would make a mess or not.