I usually put kibble, cut up chicken bits, or cut up chicken hotdogs in them, then seal them with either peanut butter or squirty cheese, and freeze them overnight. There's lots of stuff you could stuff them with, though. Here's a chance to be creative!
once they are almost empty mine will gather the kongs and stand either on the furniture, or at the top of the staircase, and heave them in an attempt to get that last bit of stuff to fly out. consequently i have red smudges on the bottom foot or so of most of my walls.
but they looooove their kongs. they know the best f the best things come in them. they get them a few times a week, ono longer everyday. and boy do they get excited when they see me digging them out to fill them up.
I mix up a bit of that all natural Laura Scudder's peanut butter (you know, the kind that you have to empty into a bowl, blend up the oil with the peanut butter, and then put it back in the jar and then refrigerate) with dry kibble...Zuke goes crazy for it!
I use the Laura Scudder's (I think that's how you spell it) because it has no sugar, just peanuts and a little salt :)
I've got 2, one for my 11 yr old GSD, & 1 for my 9mos old GSD pup. They both like them, but the pup w/stay w/hers longer, then when shes done go get his. I stuff em w/liver treats, break up some jerky treats, break up some cookies (from the dog bakery), then seal the end w/the squirt cheese. I have used the kong squirt stuff, but its over $8 a can, & didnt seem to last that long. But they do LOOOOOVVVEE them.
I didn't know that you could give dogs cheese. The only people foods that I given Mason are the cheerios and peanut butter and that's only because I asked the vet first. I'm so scared to give him any other people foods.
I bought a Kong for my puppy too because I kept seeing that toy pop up in books! Right now I stuff peanut butter in or a little bit of nutri-cal for a quick treat. All your ideas sound interesting but I am always hesistant in giving my puppy too many variety of food.