What do the stairs lead to? Basement playroom? Bedroom? Main floor?? Either way I think I’d wallpaper that whole wall. And do a small built in bench. Maybe some shelving above it.

Do you mean that closet that is under the window? Put a door on it.

Yeah home is still under construction
Stairs lead to bedroom
Door is coming there for linen closet

So what do you need help with?

The OP said "open space under window". She means that shelf! The dust collector.

OP, my sister had one of those and had a decorator fix it up. It was a beautiful spot, but a chore when it needed dusting. They had the surface finished out in hardwood that matched the stairs.

Finish the top of the shelf area with hardwood. And try to figure out how to keep it from collecting dust! (Perhaps, if you have some sort of hanging fixture over the stairs, you can dust the shelf when you put up a tall ladder to change your light bulbs!)

You can put some decorations there but yeah dusting the area would be a hassle and not possible without a ladder though.