How do you decorate a stair wall in a split entry home?

That looks like the perfect wall for a Gallery wall! Collect a bunch of your favorite pictures and things you love and lay them out on the floor until you have it just the way you want it and then hang them up!

I would hang a big mirror there.

Choose a large picture based on your style .Gallery walls tend to look cluttered

I would put a big clock there.

For ease and decoration, I have often used practical things like hats, scarves and such to decorate a wall. These might help.

https://www.google.com/search?q=decorating+walls+with+hats+and+scarves&rlz=1C1BLWB_enUS700US700&source=lnms&tbm=isch&sa=X&ved=0ahUKEwji_6WW5qrZAhVIllQKHSHXDHoQ_AUICigB&biw=1422&bih=629&dpr=1.13

A traditional way would be to line up photos or paintings going in a line upstairs. I’ve always wanted to start with a large painting on the bottom, then a medium, then a small one, then a tiny one on top. I think it would trick your eyes into believing it was a much taller staircase. ☺️

How many stairs are there on the wall you have posted a picture of? I have a gallery wall on my stairs but they are a full floor set of stairs. I never make a plan as (esp on a stairway wall) the space is far different than on a standard room wall. And also I added to mine wall over time. The theme for my wall is "pictures of females" This is the top of stairway.

Here is what I did with my split foyer entry. While you may not want to do the same thing, it might give you some ideas.