Pokémon Diamond and Pearl/Resort Area

There is not much you can do here, as there is no Mart (lamented by one lady you can talk to), so the only valuable thing about the place is the fact that you can heal yourself from ventures into Route 228. There are also a few berries you can pick, and also some valuable items the locals give you. But head out to Route 228 as soon as you are ready to do so.

Points of Interest
You can only enter this mansion (located to the very left of the Resort Area's main buildings) after you have 10 different ribbons accumulated between all of your Pokémon currently in your party. The lady at the door will step aside once you have met the requirements.
 * Ribbon Club

There are two floors to this building. The main floor of the building is the place to buy ridiculously expensive ribbons that do absolutely nothing (bet you're loving the place already). The second floor is the place to get a professional massage for absolutely free (at least something is).

Ribbon Prices
These ribbons have absolutely no effect, except the fact that you can brag to your friends when you buy one.

The second floor's massage room has the same effect as having a massage in Veilstone City, except you now have two places to get a massage. They will do a free massage once a day.

North of the Pokémon Center is a Pond. If you fish in that pond with the super rod there a random Magicarp from level 1-100. This makes it the weakest and strongest pokémon to be found in the Sinnoh.
 * Fishing