Not every wall needs to be decorated. You could easily put a large tall plant in the corner next to the window. Home Depot usually has good ones for not too much. Any of these would work well. Every room needs a plant. They're great space fillers without looking contrived. They really cozy up a room.

You have two nice features; the fireplace and the brick wall, plus a nice bit of light. Painting those dark blocks outside white will enhance the brightness and look cleaner overall. Then paint your walls that taupe/greige of the mortar on the brick wall. I think that color will work well with the fireplace too. Try two 3-seat sofas and two swivel chairs in a "U" in the middle of the room. The swivel chairs will close the bottom of the "U" and can turn to the fireplace or the piano. Be careful not to buy big puffy oversized seating. Your rug may work fine. Replace it last. I don't see any electrical outlets for lamps, but you need some. A chandelier on a dimmer will work for your ambient lighting, but you need task and accent lamps too. Shop for a piece of art 75% as wide as the piano to go 5" above it.

Ignore tv unit, matching one arriving this month

Hello! I'm a home stager and to prevent the room from looking over crowded, you can put the lounge chair next to where the bookcase is. If not, a simple rectangular bench will do: https://amzn.to/2Rk3uhM Place a few throw pillows on top of the bench and sit them upwards (and lean them against the wall at an angle) to add some elegance: https://amzn.to/32oWAhz Over by the bookcase, a faux fiddle leaf tree by Nearly Natural will be nice and their quality and shipping is great: https://amzn.to/2FzyxU9 Where the books are, I recommend canvas art or a large mirror in that space. I would remove those books because they look misplaced. The white and tan combination would be beautiful and the comment above about adding a rug with blue hues (preferably navy blue) would compliment the space very well. This one is a great price because 8 by 10 rugs are usually very expensive: https://amzn.to/33m8mZr At Home has some fantastic area rugs as well and they don't cost too much. It will look beautiful afterwards! #affiliatelink