Level select:
Hold L and quickly press Z, C-Up,
C-Down at the level selection screen to unlock every level.
Mirror levels:
Finish 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:
Finish 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.