Bowser's Gnarly Party Board:
Beat Story Mode once with any character on any difficulty.

Unlock Mini-Game "The Final Battle":
Beat Story Mode once with any character on any difficulty. The game will be found in the ETC menu.

Unlock Expert Difficulty:
Beat Story Mode on Hard difficulty with any character.

Mini-Game Room Unlockables:
- Clear a 40 piece puzzle in Jigsaw Jitters in 3:00 or less to unlock the Toad Plate
- Clear 15 pages or more in the Bookworm (or the Challenge version) minigame to unlock the Goomba Plate
- Clear Dungeon Duos within 1:30 to unlock the Shy Guy Plate
- Dive for 7 or more seconds in Take A Breather to unlock the Boo Plate
- Clear Right Oar Left? within 0:45 to unlock the Koopa Plate
- Clear Mario Medley within 1:00 to unlock the Bowser Plate
- Clear a 30 piece puzzle in Jigsaw Jitters in 2:00 or less to unlock Toad's Teapot
- Clear Beach Volley Folley's Tourney mode to unlock the Birthday Cake
- Knock down 100 or more dominos in Domination to unlock Goomba's Clock
- Clear Mario Speedwagons in 0:10 or less to unlock Boo's Photo Stand
- Clear a 20 piece puzzle in Jigsaw Jitters in 1:00 or less to unlock Toad's Cup

Taunt:
Press L during game play to hear your character's taunt. You can only do this while on the game board, not in mini-games. Press L + C-stick Up for a faster version. Press L + C-stick Down for a slower version. Press L + C-stick Left or C-stick Right to vary its volume.

Thanks to Revolution readers silly dude, Aaron Tallman, Splint15 and Game Freak64!