4. Books. Not many of us have enough books to fill an entire library. A hallway is a great location for a smaller library. And, since books don't take up much room (front to back), it can be easy to fit one in.
Tip: The shelves don't always have built-in. If you're going to incorporate bookshelves into your hallway, just make sure they look like furniture, rather than temporary, fold-up shelving that is so prevalent.