Pokémon Diamond and Pearl/Pokémon Mansion

You will encounter this masion in about the middle of the game. There is not a whole lot you can do here, but there are a couple points of interest.
 * 1) Secret spots. A decent amount of places to rock climb, as well as one point where you can surf to get a TM on the other side
 * 2) The trainers. They are not particularly hard, but they will give you a lot of money for beating them
 * 3) The trophy garden. Not much here, but you can catch a few Pokémon.