I usually make a "mush" with kibble soaked in water or broth.... soak it long enough for the kibble to get mushy, like the consistency of oatmeal. Then mix treats, peanut butter, yogurt, cream cheese, anything you can think of, with the mush, stuff it and freeze it. The more watery, the harder it'll freeze. If the dog has a hard time getting it out of the bottom of the kong, next time I'll put one or two biscuits in the bottom first, then the mush. If the mush is too watery I'll "seal" the ends with peanut butter or cream cheese to keep it from leaking in the freezer.
I also like to make "kong on a rope." Before stuffing the kong, stick a rope through the small hole and tie a knot to keep the string in there. Then stuff the kong, and when you're ready to use it you can tie the kong up on something to make it harder for the dog. It's a good one to do in the crate, or you can tie it to a piece of furnature or whatever. Especially for the dogs that just slam it down on the ground, now they have to figure out another way to do it! (Many times they just chew the rope off, but that's still ok.)