I don't think I'd do art per se up there. How about a pretty wall color, or subtle wall paper instead?

Honestly, I would leave it empty.

I don't think it is where you want to draw the eye… The fact that you struggle with "what to put up there" kind of makes my point. It is not a normal place to want to draw the eye.

I'm afraid anything up there will be too distracting. Too small. Not make sense. Fake plants? I don't think the look would work… Bunches of cluttery stuff? No!

You have (from what I can see of it) a very pretty house. Keep your decorating focal points at the level the people living down at "people height."

Builders sometimes want to turn architectural oddities into focal points. They invite you to decorate spaces that are a nightmare to decorate. Remember the silly shelves above the front door in two-story foyers? No one ever knew what to do with them!

I don't want to hurt your feelings---just my opinion---but I want you to feel it is okay after you try a bunch of stuff up there to just leave it empty. Let it be a neutral architectural thing. Keep people's eyes at room-level with other decorative statements…

Not the answer you asked for---but sometimes honesty can be helpful…

I hope this is the case with my comments.

Happy Decorating!

I see a few things for that space. See below. You could still put stuff on the ledges.