No furniture or shelves.

Do you already have a mirror near the door? A horizontal mirror would be great. Or horizontal art, one piece.

My recommendation is for a nice piece of artwork, not family pictures, something large and colorful or landscape. Likely will mount above the switches.

Why is there a little piece of chair rail on that section of wall? It should either be all the way across or not there at all. Any type of family pictures or art would work. Just confine your arrangement to above the light switches.

I would not put anything there.

I think the windows would look better with full length drapery. I would hang the art over the buffet a bit higher.

Yes I don’t understand why the chair rail was cut short either when we bought the house!
The picture will be raised :) just in the process of changing the dining room into piano living room

I’d put a framed (horizontal) art piece there. These look funky because of the slope of the photo, but to give a couple of size examples —

I think a mirror might have too much movement being across the staircase, as well as have too much light reflection/glare next to the window and sidelight.

If the window is correctly dressed, there shouldn't be room for anything there. If there will be no drapes and you often exit that door when coming from upstairs, a mirror where you can give yourself a last minute once over would be handy. I too question that piece of extra trim; you should extend or eliminate it.