Pokémon Diamond and Pearl/Pokémon Mansion

Though this is a good place to visit the first time around, it will be even better later on, after you beat the Elite Four. This place has the works of a good location: rich trainers, items to grab, and a good spot for catching wild Pokémon.

Items

 * TM87: Swagger
 * Great Ball

Trainers
Note that all cash earnings are with the Amulet Coin.
 * Gentleman Jeremy


 * Socialite Reina


 * Rich Boy Jason


 * Lady Melissa

Before the Elite Four
You can visit two main places inside the mansion: the owner's room and the Trophy Garden. Head to the right and go in the first room to get to the owner's room. In the it you can find the TM containing Swagger. Talk to the owner and he will brag about his mansion.

You can also visit the Trophy Garden, a place to catch wild Pokémon. You can reach it by going straight after you enter the mansion. Here you will find the following Pokémon: You can also visit the maid's rooms to get a Soothe Bell (from one of the maids on the left), a Great Ball, and a Burn Heal (found in a trash can).

After the Elite Four
After you beat the Elite Four, the only thing new is rare Pokémon in the Trophy Garden. Visit the owner once a day to find out what type of rare Pokémon can be found. The following is a list of some of the Pokémon that can be found:

These Pokémon have a rare chance of appearing. The last two Pokémon Backlot mentions are the only ones that can be found; the third Pokémon replaces the first one.