In the bottom left corner is a house with a sign outside. Inside is the sailor's wife and son. After obtaining the National Pokedex, The Sailor's son gets struck with nightmares of which he cannot awake. He thrashes around in his sleep and says "Dar... Dark... Is watching me...". To awaken the Sailor's son, you must go to Fullmoon Island and retrieve the Lunar Wing from Cresselia which will cure the Sailor's son of nightmares.
A locked house in the top right corner is a house (The Harbor Inn) that requires the Member Card to open. Before opening, you must obtain the Lunar Wing and help the Sailor's son of the nightmares. Not long after opening the Inn, an old man will talk to you, asking if you would like to sleep. Accepting this will make you have a nightmare of which you go to Newmoon Island, to battle/catch Darkrai. After catching/defeating Darkrai, you will wake up from your nightmare. Sleeping in the same bed will take you back to Newmoon Island where you can battle Darkrai again if you fled/fainted in battle.
TM48Skill Swap: Talk to a girl in the far Northwest corner in a house.