Okay, I'll start. I'm hoping the windows are about a foot apart in the middle of one 12' wall and that this room is next door to a full bath room. I'd use the room as a library AND guest room for people under 6' tall! I'd put a nice built-in platform with storage underneath across one 6' end of the room and some nice built-in book shelves floor to ceiling on the other 12' wall. This wall would also contain the single door in the room, and the bookshelves would surround the door. The platform built-in would hold a piece of dense foam cut to fit the platform. It could double as a place to lounge while reading or as a bed for someone under 6' tall. I'd put a row of decorative hooks on the window wall, so, a guest could hang a few things. When there is not a guest using the room, the "bed" would be covered to look like a seat.