Ever since I was a little kid, I wanted a secret room in my house. (Seems like Nancy Drew stumbled onto a few of those, and I was a big fan of Nancy Drew novels.) The closest I ever came was when my husband built a large oak set of bookshelves that served as a room divider in our house, and he left the pillars empty inside (one side of each pillar had hidden hinges) so our kids could have fun hiding in them.
If you like secret rooms, too, check out this group of 20 different secret rooms that people have created. So cool!