We've been really boring with the Kong lately.
I've just been putting kibble in there and sealing it with peanut butter. I bet Levi's getting bored of that. I used to be much more creative about it.
Levi does love a frozen Kong though. It makes it last so much longer, and it is especially fun for him to have when it's warmer. I've put veggies (like green beans and peas) in the Kong with plain non-fat yogurt or diluted low-sodium chicken broth and freeze it overnight. It takes him so much longer to work through when it's frozen!