Start a 2 player game. Quit 2 player mode, select free ride mode and complete the very first course. Return to 2 player mode again, and begin a game. After each race you'll be able to choose the Next Course option. You can't skip any courses, but you do get to play the special track after the credit track.
Beat the default scores on every track to unlock the Special Board. This Special Board has max'ed out speed and stability ratings.
When you complete the game with a character, you unlock a new outfit for that character. Each character has 3 separate outfits.
Complete all five standard tracks (Mountain Review, Emerald Forest, Urban Striker, Cave Slider, and Dancing Devils).
Successfully complete all five standard tracks with the top score.
Break various records in the super pipe to unlock the long super pipe.
Successfully complete the Cool Boarders 2 practice track with the top score.
Successfully complete the Secret Force track.
Complete a track with the top score to unlock a bonus snowboard. There are 18 total bonus snowboards to unlock.
Pause the game and press X + Y.