The hammock is a great idea so smaller "secret playroom" areas. This kid’s bookshelf opens to a secret reading nook. A SECRET READING NOOK. I would’ve never left. (from Everything About Secret Bookcase Doors)
Not that I have an empty closet, but i do like this."Closet Office" - Turn your un-used closet/reading nook into an office space. Maximize the little space with lots of shelving and storage *LIKE CORNER DESK/nook & shelving for office corner
Turn a kid's closet into a reading nookor a cute little hidden bed. Like sleeping in a fort every night. Love this for a guest room, for the kids. you could make the bed a Murphy style, so it could still be a closet when needed.