Paper Mario Cheats

To give Luigi a real scare, exit Boo's Mansion BEFORE getting Lady Bow. Return to Toad Town (You still have to fight Jr. Troopa, darn it), jump into your Pipe, and return home. Enter Mario's room and look for a strange-looking part of the floorboards (Entrance to Luigi's Diary). Go through and Luigi will be writing in his diary. He sees you, becoming stunned for a moment, then fleeing.
For fun only. Does NOT affect storyline or Luigi's Diary entries.

In Boo Mansion, go through the door across the hall from the entance, the one with a large jar in it. Jump in the jar and NES Mario pops out! Leave the room or jump back in to change back.

First, get the Super Boots. Then, go to Mario's house via Toad Town and stomp the ground inside Mario and Luigi's bedroom. Eventually, you'll see a tile on the floor going up when you stomp. Stomp on that tile and the floor will bust open Luigi's secret basement, and inside is his own secret diary!