Paper Mario: The Thousand-Year Door (GCN)
Equip both the L Emblem and W Emblem badges to change into Waluigi's clothes.
Mad about Boo
In the Pianta Parlor, go behind the female Boo's counter and she will tell you more about herself. She'll have more to say after you get high scores in the paper skill games.
One room in X-Naut Fortress' Sublevel 2 has an open vent on the ceiling. Use the Spring Jump technique to get up into the shaft. In the shaft, drop through the rightmost grate, and you'll land behind the curtain in the changing room. Walk out, and Mario and his partner will have turned into 8-bit sprites, looking like they just came from an NES game. Switch your partners and experiment with the L Emblem and W Emblem badges for more 8-bit fun. The effect will go away when you leave the room.