Go to the room that has six cabins in it. Enter the bottom right one. You will see a tile flash; examine it and you will get the Rm. 1 Key. Don't forget the Water Stone on your way out.
Room 1 is the lower left room. Enter and two tiles will flash. Examine the tile above the hole for the Rm. 4 Key (the other one is literally just trash). The other item in here is TM18, Rain Dance.
As you would suspect, Room 4 is the upper left room. Enter it and examine the trash can for the Rm. 6 Key.
Room 6 is on the far right, top side. Enter and get the Luxury Ball, then go into Room 5 to the left. You'll see a door you can't get to from here, and a bunch of nothing. To get the tiles to flash, you'll need to enter Room 5 from outside. Never fear, as there is only one item here. Stand two tiles below the bottom trash can, face left, and press A to get the Rm. 2 Key.
Room 2 is the middle room on the bottom. Go in there and you'll find just one item, the Scanner.