Get the high score on all 5 standard tracks.
Beat the game with any character to get more clothing for that character.
Cool Boarders 2 practice track:
Beat all 5 standard tracks.
Play as Snowman:
Get the high score on the Cool Boarders 2 practice track.
Play as Gray:
Beat the Force track.
Longer Super Pipe:
Break various records in the super pipe.
Play All Tracks:
To play all tracks you must start a 2-player game. Then quit 2-player mode and play free ride. Beat the first course and then quit free ride. Then go back to 2-player game and after each race you'll be able to choose the "next course" option. You have to play them all in a sequence, but you even get a special track after the credit track.
Thanks to Revolution readers Marco Anneti and Rob LaCasse!