• Another Mansion (Ura-Yashiki)
Rescue Mario and finish the game once. You will get ranked according to how you did and receive a letter grade. Save your game. You now have the option to restart Luigi's two ways (in addition to going to the completed Gallery). The first option lets you go back to the regular Luigi's Mansion Mode. The second option, called Ura-Yashiki (Another Mansion) in Japanese, lets you restart the game -- and presumably some of the things in the mansion have been changed around.

• Secret Treasure Rooms
The first one is located on the first floor, in the south-west corner of the Mansion. After you have sucked in the candle-carrying butler, exmaine the room with your Game Boy Horror to find a mousehole. Press A to activate it and go inside to enter the trasure room.
The second treasure room is accessible from the roof top. Use the elevator in the toy chamber to get on the roof, the go all the way to the right and climb up on the chimney. Fall down and you will land in another treasure room.

• Water the Plant
Go to the right side of the graveyard. just underneath the balcony with toad. There is a seed. Once you can use water (BEFORE the second boss) water the seed. enter the doghouse, and defeat the second boss. return to seed and water again. continue with game, but come back and water it after you fight the 3rd boss. It will bloom and produce a diamond!