I usually do PB also... It's Rook's favorite. He also loves apples, so I will sometimes core & peel an apple (the seeds are definitely bad, but i think the skins are actually okay... I'm not sure why I started peeling them) and cut it up into small chunks to stuff in the Kong. It keeps him busy for a while try to get them all out.
Sometimes I'll break up some of the treats I've made for him, or we'll put a chicken broth ice cube in it along w/ some regular kibble.
At bedtime, we actually put some of his regular kibble and "bind" it with the Kong EZ cheez stuff. He knows it's bedtime when we do this and he literally leaps into his crate when he sees the Kong cheez can