It’s probably not a good idea to use shelves that you can’t reach for storing items that you need often. But if you’re looking for out-of-the-way space to store knick-knacks, a shelf between the top of the door and the ceiling might be a good place for them.
Consider continuing the shelf all the way around the perimeter of the room. That will give you plenty of display space, plus it will give the room a designed, finished look.
Paint the shelf white or a shade or two lighter for contrast. A dark color might cut the room off at the shelf.
One note: It’s hard to clean anything you can’t reach. Keep a footstool handy so you can regularly dust your tall shelves and their contents.