One of my major life goals is to own a home with secret rooms and passageways. Ideally, I would like to build such spaces before my children are old enough to be aware of architecture, so they will grow up believing that it is common and normal for buildings to have magical and secret worlds within them.
If you have a somewhat boring-ish passageway that you'd like to put to use, make it a secret bookcase. You can work thick walls to your advantage this way, or you can furr out the wall a bit (or even part of the wall, as seen in this photo) to add a little depth. (This would be a clever way to divide a large shared kids' bedroom space as well. Build a wall with a secret door!)