I'd say one piece of art about the desk (quite large) and maybe add a plant to bring in some life and green. Not sure where you'd put it as that is up to you but I'd say a desk plant with substantial height to add some more to the wall space

Open shelves.

Yes, shelves.

What do you want to look at while you're using your office? I would hang something that makes me feel good on the wide wall. On the sides, I'd put useful things...shelves?