Let's start with the basics. This room is a great space, you have beautiful flooring. The paint color Is a bit dark It makes the room look dreary because their is not a lot of natural light. Try something lighter. Paint is cheap and easy to put up. You may want to change the couch it looks a bit out-dated or just but some bright accent pillows. Select a color scheme for that room and it will surely look Awesome!

I agree with @acm I forgot to mention area rug ;)

Center the sofa on wall opposite the window. Add end tables height sofa arms or slightly higher, matching lamps in silver or reflective light, a 9 x 12 or 8 x 10 rug running long side parallel to window with front sofa legs sitting on the rug with the blues you seem to love. Hang large art or mirror, 2/3 width of sofa, over the sofa about 5-10 inches from top of sofa.

Center piano on other wall with piano lamp and some type of greenery on or nearby the piano.

Bring in some types of storage for the toys....large bins or baskets that will give a clean look when toys are stored away.

Here are some rooms with the elements I think could work in your room for a more styled look. Do not look at the design but the element and interpret it to your style and what is out there for buying.

Art over piano with flowers and chair

Mirror and music lamp

Lamp and table with lamp same height as mirror and sofa at right height over sofa. throw pillows, covered table for hidden storage, basket under cocktail table for storage.

Again, end tables, greenery, area rug, pillows, art work and mirror in same room, cocktail table.

Again elements of area rug that is big, round cocktail table safe for little ones, storage in drawers, pillows, end table and lamps

Modern elements, in grid mirror, spiky plant, large lamps to balance large sofa and mirror, area rug, art work in colors similar to the lamp and sofa.

For its health, the piano needs to be on an interior wall, so move it opposite the window. Ditch the sofa for a grouping of 4-5 chairs around a hassock or coffee table, like this, but your piano will take the fireplace spot. The chairs don't have to match; you can pick them up second hand. Hang a big piece of art above the piano and on the other big wall. An area rug could set the tone for a new, brighter wall color.

Great examples Susan Davis. Also agree with adding art. This is very hard for another person to choose your art though. This is a very personal thing. Choose something that is pleasant to look at & perhaps relates with the overall look of the room.

What is the budget for this room?

How do you want to use the room? Obviously piano playing. Quiet reading? Conversation? TV? If so, how many people? Children's play area? Is anything staying in the room in addition to the piano?

Please stand in each corner and take a photo towards the opposite corner.