Secret passageway! Cute idea if your house is designed properly. I remember a family friend's house had a hidden play room under their stairs. To get in, we crawled through a little door in their coat closet. Way fun for kids!
just sparked an idea of a new dream home necessity: small hidden spaces (amount depends on size of dream home) that serve as emergency hidding spots in case of intrusion/burgleries,, equipped w a direct 911 source
bookshelf door. Not only because it can basically be a secret passage way, but because it's a book case!