You want to showcase the beauty of the piano, but not clutter the area.
Framed Music
One unique and budget-friendly way to decorate a wall above a piano is to frame and hang sheet music. As pianos are rather large pieces of furniture, hang at least three framed pictures of sheet music above the piano to provide balance and scale. It looks best when arrangements of pictures and items are in groups of three or another odd number. Sheet music is black and white, as are pianos, so frame the music in black, or in a brighter color like a strong red to add pop.
Wall Art
Choose a large piece of art for the wall above a piano. Placing a single, smaller picture in this space will make it look out of balance. A landscape or city skyline would look lovely hanging above a piano.
Mirror
A large mirror is well-suited to a wall above a piano. It's fairly common to hang a mirror in that spot. Use a substantial mirror with a very strong frame. It should be hung a bit above the piano, but not too close to the ceiling. The actual frame around the mirror doesn't matter. Find something that suits the colors and style of your home.
Sconces
Wall sconces are a lovely decorative feature for your home. Sconces are mounted on the wall and generally have a mirror or a candle holder. They come in all kinds of colors and finishes. To use on a wall above a piano, mount two sconces on opposite ends of the wall. You can also use three, placing the third a bit lower and in the middle.