My quick version:
Because of the mounted tv - which is quite helpful for the rest of the living room arrangement - I often have a little trouble figuring out what to do with the mantel. There's not a ton of space to work with, and I don't exactly want anything distracting from my view of Project Runway. So I kept it simple with some 59 cent mini pumpkins, my cork collection that fits with the warm colors, a candle for cozy lighting now that the evenings are getting shorter, and of course my books.
I arranged some books on top of the shelves on the other side of the room to match. The vintage-y keys were a Michael's $1 section find.