The first thing I would do is replace the clock with a decent sized picture. Then I would scout around my house for some knick-knacks like you've got on the shelves there, and see what kind of an arrangement I could come up with. I see a lot of white china that could be grouped on the top of the mantle. You just have to rearrange it till it looks right to you--that's when it's done.

I would also consider putting some small plants such an African violets on there depending on what else I came up with.