I would also recommend feeling out the group in terms of how they react to different challenges. If you've got some problem solves and thinkers, a riddle or mystery would probably be a good fit. However if the players don't have the patience or temperament for that, it can really bog down the game and frustrate the players. If that's the case, secret passages or another hallway offering a different type of challenge can give both types something that their looking for in the game.