If you're running low on lifeforce, get out your vaccuum and start cleaning lamps and vases. Almost every second vase has a heart hidden inside -- and nearly every lamp or chandelier either yields money or health. Happy hunting.
Hidden Mansion Mode
Rescue Mario and finish the game. You will get ranked according to how you did and receive a letter grade. Your game automatically saves after the credits. You now have the option to restart Luigi's Mansion 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 Hidden Mansion (Ura-Yashiki in Japanese), lets you restart the game -- and some of the things in the mansion have been changed around. The differences are very subtle, so don't expect a whole new quest, though.
Reward for 50 Boos
One of the sidequests in Luigi's Mansion is the capture of the 50 Boos that you released accidentally in the beginning of the game. Finding Boos can be a time-consuming task as you have to make your way through the whole mansion and track them with your Game Boy Horror "Boo Sensor." Boos hide in furniture in rooms with the lights on, so be sure to check in every nook and cranny. A blue light on your GBH means there's no Boo to be found, a yellow light means you're close, and red means you're right in front of the hiding ghost. If the light is red, switch on your vaccuum or examine the object in question with A. Watch out, though, not every red light equals a Boo. Sometimes a Boo Ball will fall out (you can simply blast it at a wall or leave it where it is), or a bomb that has the potential to cause some damage -- so run! If you're having problems finding all 50 Boos, be sure to look in the "door-less" treasure rooms.
The reward for catching all 50 Boos is a large diamond.
Secret Treasure Rooms
There are two secret treasure rooms in Luigi's Mansion that you can access to load up on gems and money.
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.
Secret Warp Zones
Running around from one end of the mansion to the other can be a very time consuming task. Thankfully, Nintendo has implemented hidden warps that let you zoom back to the entrance hall.
Basically, any mirror in the mansion (except for the large ones that reflect invisible ghosts) can act as a warp. To warp using a mirror, switch to your Game Boy Horror's first person mode and examine the mirror with A. After a really trippy warp sequence, you will find yourself in the entrance hall of Luigi's Mansion.
Water the Plant
Ever notice the seed in the outdoors area with the dog house? It's at the right side, near the graveyard and just underneath the balcony with Toad. This seed hides a secret that you can unlock right before facing the second boss. Once you have the ability to use the Water Element, water the seed. A plant will grow. Now enter the dog house (examine it with the Game Boy Horror) and defeat the second boss. Return to this area afterwards and water the plant again. Now continue with the game, but be sure to return after you defeat boss #3. Water the plant again and it will bloom and produce a precious diamond.
Note that if you fail to return and water the plant after a boss fight, the plant will whither and die.