I think a table or long surface near the lazyboy-like recliner would be good. You don't want to do anything that will kill the light but some demarcation would be good.

I'd put the lazyboy opposite the couch to make a cosy seating circle. If you need a bookcase, it would go nicely between the couch area and the piano. Either flush against the wall or use the half height ones to make a room divider like this one from west elm http://www.westelm.com/online/store/ProductDisplay?partNumber=WE-PRODf519&storeId=17001&langId=-1&catalogId=17002&viewSetCode=E&parentId=WE-SH1FRNSTO&retainNav=true&cmsrc=WE-SH1FRNSTO

If you dont have a seperate dining room you could use the space in the middle to set up a dining area....

what is the big black thing on the wall opposite the credenza? Is it a storage unit of some kind?

With so much lovely light you could probably put a small indoor tree somewhere also....

Note that all postings Anne07 has made to AT include suggestions for products at that site....(not that thats bad-I dont believe you need a divider, since you want the light to reach through the whole room.
Also = Do something fun to highlight the fireplace perhap?

thanks for the ideas! The "How can we break up this space" wasn't actually my exact question... I'm more worried about designing the space with different furnishings, moving things around, changing the paint color, getting rid of the sideboard (the big black glossy thing across from the piano) and replacing it with a new one, adding bookshelves at the far end of the room, and adding light sources as it is a very dark room. Thanks again for your help!

You definitely need to change the paint color. Go light, like a gray. That will look great with your floors and moldings. I'd add thick baseboards as well!

I like the idea of a long table as well. A space for eating, and a work space. You can have one end for keeping your laptop and the other end for eating or reading. I think it's important to keep your furniture off the wall in this space.

This piano is beautiful. Maybe consider getting a small bench instead of the stool and covering it in a great print.

I wouldn't advise going the screen route. Instead, I'd divide the space by using area rugs. A large one under the long table and a smaller one to ground a seating area.

good luck.

I'd go with area rugs too. One for a living/piano space. The other for a dining/game table space. You can also use paint to alter the look of the two spaces. I'd use a darker version of the same color in the window area. Then stripe a few feet between with a lighter shade. Then go toward the door with the lighter shade. I would not use a divider since you may have trouble getting enough light at the door side of the room. Mirrors would be good here to bounce the light back to the dark part of the room.

i have a very similar problem (except my one big room is a studio and also includes my bedroom). my question is, is it possible to paint the room two different colors to divide the sace? if so, how would it be possible to make the separation? or does the whole room have to be the same color?