How to decorate a mantel?

It doesn't look like there is much room on either end, but you could always add some pottery and small plants to bring in some color?

maybe a vine type of look that runs up the sides and extends into the upper space. It will look better f you draw the attention away from the tv,

find another place for the TV.

couple of tall vases with glass balls/beads in them. Maybe tall grass/birch branches Fabric panels along sides

I would take the TV down. Over the mantle is not a good place for a TV and heat can ruin the screen.

There's room for a nice mantle garland that can be draped down the sides a bit. Pick some nice slender decoration for each side of the fireplace legs depending on your theme. For example, a pair of tall, slender nutcrackers would look great. Tall narrow vases with berry sprays or lighted twigs, a selection of wrapped boxes in different sizes, an arrangement of pillar candles in front of the fireplace itself to lessen that black hole! There are lots of options but as I said it depends if you have a theme you want to follow. You can also play a dvd of christmas images on the screen when you're not watching tv itself, almost like a moving art gallery!

How about possibly adding a second mantle shelf a about halfway down the mantle front above the opening and then adding your things like plants, display pieces, etc.

I think moving the TV is the best idea.

This TV is way too large don't bother with the mantle the HUGE TV will dwarf any effort to fix up the mantle.