Get Gallery Mode:
Finish the game to get gallery mode

Go to Main Entrance:
Use a Game Boy Advance and look at the mirror in the rear of the gym. Now on the Gameboy Advance press A while facing the mirror in the 1st person

Reset Game:
Hold B + X and Start for 2 seconds

Get New Mansion:
After finishing the game, save and you will have the option unlocked

Surprise the Secret Ghosts:
Have you noticed that in some rooms there are ghosts that won't appear until you turn your back to them? Well in one room where you meet the first one, there is a shelf of books that
explain that you should wait a few seconds and pretend that you aren't doing anything and then turn around with the flashlight on and stun the ghost so that you can press the R button to suck up the 100 lifepoint ghoul!

Easiest way to catch the Big book reading Ghost:
When you go to the room where the books fly at you go to the fireplace and start sucking the vacuum slowly walk backwards with the vacuum still on when you get to his tail like thing he will get sucked up and just hold on!

Get reading ghost:
This made me very frustrated and i could never figure it out, but I finally figured it out a day or 2 ago. There is this one room where there is this ghost and he is rocking in a rocking chair and reading a book (I think it's a book). Anyway, leave your light off because it won't effect him. Wait for him to yawn, then hit him with the flashlight! I don't know why the designers made him vulnerable to the flashlight only when he is yawning, but just like the tootsie-pop commercial says "The world may never know...."

The Big, Bad Boo's Attacks:
In the start of the game, Luigi releases 50 Boos. Yes, 50 Boos. To find the Boos, search lighted rooms with your Gameboy Horror in the corners. Yes, in lighted rooms. A green light means there's no Boo, yellow means you're close, and red means you're right on target! Press the A button to examine the area and hopefully find a Boo. Your reward, a large diamond!

Water the Diamond Plant:
Below the balcony where Toad is there is a dog house and a plant where you can get one of the game's secrets. Once you have the water element go to the plant before you face the second boss and water it. Then, return and
water the plant after you beat the second boss. Then, once more, return to water it after you beat the third boss. Your reward is a large diamond (Note: if you fail to return and water it, then the plant will wither and die).

The Hidden Hearts and Money:
If you are low on life energy, then get out your vacuum and suck in any lamps, chandeliers, or vases. Almost every one will have money or hearts in it, so do it often!

The 2 Treasure Rooms:
There are 2 treasure rooms that are hidden pretty well. The first one can be found on the southwest corner of the mansion on the first floor. Use your Gameboy Horror in the room to find the mouse hole and then press the A button to go in to the treasure room. The second room is hidden so well that you can only access it from the rooftop. Fall down the chimney and you'll be in the last treasure room!

Extra Treasures:
When you are sucking up a large ghost (Like the one sitting in the library), he will drop more treasures the longer you hold on to him. If you manage to suck up 90 of his HP continuously (note CONTINUOUSLY), he will drop EXTREMELY expensive treasure worth up to $10,000!

Ceiling Fan Secret:
In the room with the ghost brushing her hair, there's a ceiling fan. Use your vacum on it. By now, you would of thought that there is nothing in it, but if you make it goes faster and faster, lots of treasure will come out.

Hidden Hole:
There's a mouse hole on the east wall of the butler's room. When you select the hole with the Game Boy Horror and press the A button, you'll earn passage to the treasure-filled hidden room.

How to Catch SuperModel Ghost:
In the Master Bed room there is a chair in the corner by the window, if you turn your back to it or look at it through the Gameboy horror, you will see a ghost that has a red dress on, if you go over to the window and pull the curtain back, she will go over and close the curtain back, and that is your time to suck her up.... OH yeah watch out for her mushroom things!

Mirror Trick:
Go to any mirror, than use the Gameboy Horror and do a search directly at mirror. Should take you to Mansion Lobby. Good if you want to save and you dont want to travel through ghosts and stuff.

Beat The Big Baby:
If you are trying to beat the baby Chauncey then, here is your answer. Ok first, go to the rocking horse in the back of the room. Then, start sucking the rocking horse so that it begins to rock back and forth. Chauncey will appear over top of the horse and ask if you want to play with him. As soon as he starts attacking, suck up the ball so that it gets stuck on the vacuum, then, keep sucking until you are aiming at Chauncey, finally, stop the sucking and switch to blowing, the ball should shoot at Chauncey, after that he will turn Luigi little. Avoid the rocking horses and when he starts throwing balls at you. When one hits the back of the crib and bounces to the front, suck it like you did when you were normal size and shoot it at him, if it hits him then aim your vacuum up a little bit and when his heart appears, start sucking him in, if you don't get him the first time try again, before he starts throwing the rocking horses, he will cry and start pounding the ground with his stomach, avoid this and repeat the above. When you have finished him off, you will be transported back to the lab.

How to beat Bowser:
When bowser throws his spiked bombs at you wait for them to stop moving. Then pick them up with you vacuum. Then you have to wait till bowser lowers his head to breath fire or to breath you into his mouth and shoot it at his mouth the head will pop off and boo will pop out now turn on the vacuum and lower his health points while avoiding the ice blasts that comm from bowsers head. Repeat this process and you will beat bowser..