Although it is often met with some reservations, a great way to display artwork is to hang it in front of bookshelves. It is unexpected and helps breakup what can be a visual cacophony of book spines.
The two most common worries are: 1. But I can’t get to the books behind the painting/photo and 2. I don’t want to put a nail hole in the shelf. The solutions are quite easy. For book access either remove books from behind the artwork or put books that are rarely used (plus the artwork is not permanently attached if you need access). As for the damage to the shelf, the holes that picture hooks create are quite small and can easily be repaired (a tiny drop of wood puddy on one’s finger will typically do it).
So if you have run out of wall space and still have things to display try looking where you might not expect to see artwork. It can take the task of hanging a picture to the installation of a prized artwork.