I'd wallpaper it--grasscloth would be pretty. Add some color--it's very neutral and blah right now.

I feel that picture molding will look a bit "off" as it will be viewed almost strictly at an angle.
Wallpaper or tapestries may look alright.
I would avoid hanging framed art work...in confined spaces that serve as passages these items can get snagged ,bumped and otherwise misaligned.
Then there is the additional task of keeping the pictures dust free.

What about adding a fun wallpaper (right into your open office as you were going to paper anyway). Given your art, I thought you might like a bolder look.

Could be stunning.

Here is board and batten detail on the staircase walls

I like the idea of using a fun wallpaper shared by @Maureen. Find a wallpaper you love and use it on the stairway as framed in art and then use the same wallpaper for the upstairs office. Here's an example.

Wallpaper or a gallery wall would be very nice.

You might also change the carpeting for a striped runner.

I redid my staircase wall when the plaster crumbled and the entire wall had to be replaced with drywall. I installed grasscloth and then did a family photo gallery wall. I also replaced the old worn-out runner with a cotton stripe.

I’m with Diana. My long hallway became the best place for framed family photos. Your stairwell wall is a good place for those.

If you decide on a gallery wall, work with the railing and test configurations. This will be easier and less expensive than wallpaper if that was a concern.

It can be photos or a colourful eclectic combination (then duplicate the colors/feel in your office wallpaper).

Molding just pluncked on a wall is never good odea. I for sure would hang a piece of art at the top of the stairs tio draw your eye and it could be quite large too. A great light fixture is also a must.

My first suggestion is that you add lighting. The stairwell looks rather dark in your photo.

Thanks for the ideas, everyone!

To respond to a few things:

Lights are off in this picture as it's the middle of the day and we usually don't need lights on. I was planning on swapping out the puck light in my "office" (technically just a small loft) for a fun fixture. Our lighting is kind of centered over the railing, not the stairs, so I would have to do a bit more work to add a dedicated stair fixture (hire someone, move electrical, patch the ceiling, $$$). Was hoping to DIY this.

We don't have a view of this wall from any distance except from the hallway above, so I was kind of shying away from family photos or a gallery wall. We have family photos opposite this wall currently, in the hall. I felt similarly about a large painting - I just am not sure how it would look from the views we would have (bottom of stairs vs hallway). Just seems like it would look poorly placed from one or the other. I was thinking molding would provide something other than a blank wall but not feel forced like a gallery wall would but I guess I'm not understanding why molding isn't a good idea - what makes molding in the photos Beverly posted OK but not OK for my stairway? I'm seriously asking. My home is more transitional.

Our stairway is not constructed for the carpet to be removed, so taking it off and adding a runner is quite a bit more challenging and expensive than it sounds (we'd need to rebuild the stair). I do have neighbors who replaced their carpeting in their whole house with LVP but still left the stairs carpeted due to the expense.

I do like the idea of framing wallpaper, if molding is a no-no. That might be a good way to tie into my office but not have art or photos that would require people to actually look at them. I have drapes I made for my daughter's nursery that I wouldn't mind hanging in my office that are geometric yellow, so I could tie that in.

@Maureen The wallpaper you posted with the pink & blue flowers was one I had saved! But then I thought maybe I should go for a bit more drama (and use the drapes I have). I'm still undecided.

I'd keep wall adornment to a minimum because of the narrowness of the space. I'd hang a nice pendent or chandelier into the stairwell from the second floor ceiling. And, I'd get interesting carpeting or a runner for the stairs. You clearly have a good sense of style and like color and pattern so I think you could make this an interesting space with just the right fixture and floor covering.

That’s a coincidence! Might was well complement your drapes if you like them.

@Maureen These are the curtains we made for my daughter's nursery (didn't know when we made them that grommet tops are a no-no, but they are blackout, very heavy, and a labor of love). She doesn't want them in her room now but I would still like to use them. My office is probably the only spot in our house that we could.

So, I was considering this wallpaper just behind my desk, which I thought might go with my curtains, and I'd still be able to keep my blue office chair.

There are so many beautiful papers and I’d try to complement the chair, drapes and the wall color. Not as bold, but would be stunning on the one wall.