I had the same fascination as a child, and I still do as an adult. I remember designing these hidden areas in my imagination on certain nights. I really like the idea of a bed that slides to the side revealing a staircase to a basement. I also dig the idea of a poster or tapestry on the ceiling that when removed reveals a secret hole in the ceiling that has a drop down ladder to an attic. I also very distinctly remember wishing that I could make a secret passage that connected the closets of multiple rooms. The book shelves are still so classic, and I love the metaphor that it provides. Books are the key to other worlds :)
Maybe one day I'll be a crypto millionare and design my house with all sorts of secret lairs.