We live in an old house that had quite a lot of fireplace mantles, but they originally had oil stoves in front of them, so no fire, just the mantles. I have covered them up, put painted panels in them, among other things. But this is the idea I like the best.
This is also a great idea to do if you don't have a fireplace but want the look of one.... Find an old mantle at a flea market. Put a piece of 3/8 plywood onto the back of the fireplace as a base for your wood fill.
It is then really quite simple. Find an old tree that needs to be cut down - or get unsplit firewood from someone. In this instance, we used a birch tree that had died on our property.
I had my husband cut the logs into slices between 1" and 2" thick. Make sure to cut even the smaller branches, as you use those to fill in.
With a nail gun, start in one corner and just start filling in the open space. One nail per log piece should do.
Here are a couple shots of some freestanding mantles that I did.
They make great decor pieces. This larger one could also be used as a headboard.