What about wallpaper?
Molding treatment?

This is what I would do

do you collect anything? for example, I collect antique cameras and that ledge would be perfect for that.

or if you have no idea, maybe find a long sculpture. or a collection of objects (vases, bowls, etc) in different shapes and sizes but with a cohesive look. don't rush out and buy a bunch of decorative stuff from a discount store, you will be looking at it all the time. wait for something special to "find you". take the pinecones down for now, this shelf will look fine blank while you look.

be sure whatever you pick is easy to dust using a duster on a long pole. if the items are light weight, I recommend using "museum putty" so that your duster doesn't accidentally knock the objects around.

also, UGH to sellers leaving "gifts" because they're too lazy to take care of it. if they can put it up, they can take it down. super annoying.

How about hanging a great piece of art? Not a framed picture which might either fade from window light exposure or have lots of distortion from glare from window but thinking a large metal or wood piece. Seems that when coming down stairs that’s perfect vantage point. Also looks like there’s an HVAC return or vent on stairwell wall so might be challenging to hang large art on that wall. @palimpsest idea of walling off the ledge is good too but then what? Still a good place for art & cheaper just to do that!

yes, that space would frame wall art nicely. either framed artwork, or it could be objects.

make sure it's big, or it will look dinky and awkward in that nook.

I was thinking of wall baskets! Love the wire art too

The wall, or a nicely detailed shallow niche (and I don't normally like random niches) would be a good place for art, I agree. It's the actual ledge thing that I dislike, being able to look down on the dust it accumulates or the regular awkward cleaning it requires.

the wire art is from West Elm. I love it too! it might be too small for your space, but here's the link in case you want to check it out!

@Ayla W - Budget permitting, I would choose something metal over anything woven. @User first pictured suggestion of the metal circular & woven looking metal hanging is a perfect compromise. My reasons for choosing metal is it has a cleaner look - literally. Baskets or woven things just look dusty really fast & that's not a spot you're going to be able to access easily to dust. Also, metal art will give you more color options & you definitely will acheive a nicer look with some color in that spot even if painting the walls a color is in your plans.

Super great point about the dust on woven items! I wish there was the option for a funky chandelier (it’s in the to-do list for later down the line) but it would require opening the wall and ceiling and running wiring… for now a big piece hung up would be great.

Take the items down, then leave it empty as an architectural feature. A Webster duster with expandable handle would make quick work of dusting this spot.

If you prefer art, measure the space while you are up there retrieving the seller's things. Then choose something you will love to look at, not something that is just there to occupy space.