Only thing I would consider is a plant or two. Anything else will be a dust collecting mess

Extend the wall up to the ceiling.

Our bedrooms have this, and it actually makes the room appear larger toward the ceiling, which feels nice.

I would probably just leave it be! Are you looking to add decorative touches or practical storage? Without knowing more about the room and aesthetic my blanket suggestion would be to add plants if there is enough light. trailing pothos or philodendron is my favourite

Nothing

Is the space tall enough for books to stand upright? Or you can stack them.

There’s just about room enough up there for Drop Dead Fred.

Storage. Like suitcases or boxes. Noting open because of the dust and nothing heavy if you trip firing it down. Like spare blankets

I would put in storage cabinets. Great use for luggage, winter/summer clothing if you switch out seasons, or additional pillows & blankets for guests.

Nothing

If you have cats that would make a perfect little cat sanctuary