One of the design features I have always wanted in a house is some sort of secret passageway. I want it to be super secret as in people-who-come-over-have-no-idea-it's-there secret. It's not a secret passageway if it has a door handle on it. Maybe it goes to an extra room, but a door handle = a non-secret room. Here are some great ideas of how to build a secret passageway into a home.
Clearly bookshelves are an easy way to hide an entrance to a hallway or room. I like to think that you could add a button or handle behind a painting to open up the wall and reveal a secret passageway.
Maybe I've read too much Nancy Drew in my lifetime, but there is just something mysterious and wonderful about a secret passageway, don't you think?