Books, books and more books! I've always wanted to have a high-ceilinged room with books from floor to ceiling. so high in fact, that you need one of those slidey-ladders to reach the uppermost shelves.
This is the perfect place to put a pull out bed. I always look at those beds and think they're impractical because it's too hard to leave all of the open floor space needed for one but I'd happily do it for this library/reading space