Tokyo Subway: The Mother of All Puzzles
Finding your way to the right station is difficult enough. But that's only a small part of the bigger challenge. Once you reach your station you have to navigate millions of intertwining tunnels with gazillions of exits. But I have to say it's been fun getting lost underground. It's a great place for people-watching and window-shopping. Plus its a good workout (no wonder people are so lean and fit here).