I had a Mantel at my old house, and was always fretting with what to put up there. I seems that it is just the hardest part of the house for me to decorate. I remember when my sister-in-law was having the same problem, and we both just settled for less is better. You don't want your mantel to look cluttered, so keep it really simple.
A mirror and a plant, or vase of flowers is perfect. Nice to look at, yet very simple and clean