Another idea is to paint part of the wall with blackboard paint. You could just do shapes or work on a city skyline that goes right around the room. Your friends can autograph the wall with chalk or you can keep lists in plain sight. A teen has lots of stuff. It just tends to collect and usually ends up on the floor because there isn't enough space in the closet for it.
Shelves are the answer.
You can make shelves out of just about anything, from cement bricks and boards to those plastic crates that eggs come in at the supermarket. Ask for the crates at the store and then stack them in your room, on their sides. They come in all kinds of bright colors, but you can always spray paint them if you like. They will add a softer look to the room. You can also hang old fabric on the walls.