Level select:
Hold L and quickly press Z, C-Up, C-Down at the level
selection screen to unlock all levels.
Mirror levels:
Successfully complete levels 1 through 4. A message will
appear after the credits will confirm correct trick entry.
Press Z when selecting a level in time attack, two player,
and championship modes to play on the mirrored version.
Bonus characters:
Successfully complete the mirror levels 1 through 4 with Skier
and Boarder. A message confirming that the three bonus characters
(robot, bear, disco dancer) have been unlocked will appear after the credits.
Sites: