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.