Secret warp zone
In the gym room, turn towards the mirror at the back wall. Switch to your Game Boy horror's first person mode and examine the mirror with A. You will be warped to the entrance hall of Luigi's Mansion.

Hidden hearts
When you're running low on life, use your vaccuum and clean the lamps and vases. Almost every second vase has a heart hidden inside and almost every lamp or chandelier contains money or health.

Capture 50 Boos
If you capture the 50 Boos that you released accidentally in the beginning of the game, you will be rewarded with a large diamond.

Secret treasure rooms
There are two secret treasure rooms in Luigi's Mansion where you can get lots of gems and money. 1.) One is located on the first floor, in the south-west corner. After you suck in the butler, examine the room with your Gameboy Horror and you will find a mousehole. Press A to activate it and go inside the secret treasure room. 2.) The other you can access from the roof. Use the elevator in the toy chamber to get onto the roof. Go to the right and climb up the chimney. Fall down inside and you will enter another secret treasure room.