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)
Hidden Doors What could be cooler than having a door that’s completely concealed when it’s closed? One so secret that until you open it, no one else even knows it exists. Hidden doors are typically attached to large bookshelves but can be found behind other seemingly ordinary things, too, like life size pieces of art.