Christmas decor aside - where the sky is the limit - I would not want to fussy up a fireplace setting that already has busy stacked stone. One large-ish picture leaning on the mantel would do it for me.

I totally agree with graywings. That stone is so beautiful and very busy; I wouldn't want to add anything that would compete or detract. A single large piece of art would be perfect.

What a beautiful fireplace, and I really like that paint color with it. Have you done a google image search for ideas? Here are a few I found:

Have fun decorating it for spring!

I prefer simple things -- a picture or a mirror, perhaps some candlesticks (whatever trips your trigger but would prefer more weighty ones) and perhaps one big "thing" on the other side.

Is there a website that sells rustic pictures/candle holders? I went to our local Hobby Lobby and found nothing...

The mantel shelf is too high and also the wrong color, too red. I would lower it, repaint it a darker color and then get a large painting, with some accent pieces on the side like you can see on some of the pictures above. The proportions are off on yours and that's part of the problem. Very nice fireplace otherwise.

Edited: Ouch is right, I'm reading this over a day later and apologize for how very brusque it sounds. I really didn't mean for it to come across like that, forgive me.

ouch!

Here are some candle holders:
Rustic candle holders, shopping search on google

You could also look up diy versions, if you like doing your own projects.

Rustic framed art

Art is so personal, what one person likes, another won't. I'd just be careful to have the art be somewhat different than the fireplace, in colors and texture. Some rustic art (I wasn't sure what that was) that comes up in laughable's link seems like it might be too similar to the colors in the fireplace and could disappear.

Not sure where you live, but also take a look at places like Home Goods, World Market, even Target, if you're not ready to purchase fine art. Some towns/cities have great 2nd hand stores and you might find something there. If you have the budget, then find artists you like and look at their websites.

Art.com and similar have so many selections and you can sort the choices by subject matter, color, or artist, etc. You can play with frames and matting, too.

By the way, I would LOVE to have your fireplace!

In my parents' house in Texas, there was a gun rack above the fireplace, but it did have my GG grandfather's Civil War rifle on it, along with his bullet mold on top of the fireplace. I would rather have had antique swords or African spears instead of rifles, and the fireplace was in my bedroom! Later, my mother moved the gun rack to the hall and replaced it with a cast plaster eagle with wings spread, which was an improvement, but I did not like Early American decor either.

I think that if you identify your style, you will find what is best to go above your fireplace.

I have nothing to add on what you should hang or put there. I can only say that it's not always about what you pick. It's often more about scale. I finally took my room pictures to a small used home accessory store. I picked out these Huge birds at almost 15 inches tall. The gal in the shop talked me into the ugly center thing that turned out to work well with the stone. I had already bought the mirror but was stumped on the rest. I'm done and spent very little money. The hardest part for me was finding things big enough as things seem to shrink once put there. Nothing under fourteen inches tall works...just looked like piddly clutter from afar.

JTL, that is so cool! I especially like how you've used that mirror. It brings some light and sparkle, but no worries about whether there is something worth reflecting like there is with so many fireplace mantel mirrors.

What is the thing where the screen would be?

Hey Oly, that bubbly metal metallic thing is a wall sculpture from my old house. I have never used and probably never will use the fireplace so I stuck the sculpture over the black rusted ugly built-in slide screen.