(Some information sourced from The Cutting Room Floor a fantastic source of information about unused/cut content from video games. Other information has been sourced directly from my own collection.)
Debug Mode
Replace w000.bin (data Folder) and w000Dll.rel (dll folder) with selmenu.bin and selmenuDll.rel. Start a new game in story mode and you should be taken to the debug menu. It's nearly identical to the one used in Mario Party 6, which is the engine the game uses.
Dance View
DANCE VIEW lets you view the dance moves of each character. For some reason, it endlessly zooms out, which can get annoying.
Motion Check
MOTCHK lets you view Mario and Luigi's animations. It has the same zooming problem.