Looks like you have a cool space. What do you use the room for besides sleeping? Is this where you study, hang out with friends, listen to music, etc? You might try dividing your room into mini-rooms. For instance you could place a desk for studying, computer so it sits perpendicular to the large window. This could act as a division between sleeping and living quarters.

Shelving may help corral your music/books/objects. Find new bedding that is brighter or add some colorful throw pillows to your bed.

As for the stairwell, use found objects to decorate the walls. You have a good start with the hats and the directional sign. Look in the shed, the basement, alleys, etc.

Good luck.

I would try the desk under the window. It would allow you to look out while using it, and give you a place next to your bed to place things, almost like a nightstand. Then you have all the space under the slope for the sofa and hanging out area. It looks like you could also use some sort of floor lamp, to give you light and atmosphere. Have fun!

gorgeous window!

Your Bertoia Bird Chair and Calder mobile are statement pieces that you can build upon. Have you considered painting the walls? The builder's white is doing you no favors. You could create a seating area with the Bird chair and the futon. Choose a nice cover for the futon in a fabric that you like. An area rug would ground the hang out space.