I helped my daughter redo her bathroom, and she got a new door. Her old door was very heavy and solid. I cut it in half vertically, added a 1×1 in the back corner, and glued and screwed it together.
I made some shelves out of plywood, and added some trim down each side. I made some small cleats to support the shelves.
I added a piece of trim at the top, and some quarter round to finish off the front face of the shelves.
I love the way it turned out, and it fits so nicely in the corner.
I also made a bookshelf from two old doors: