All characters:
You must pause the game, then hold L and press
X, B, Left, Y, Right, Right to unlock all characters.

Flying abilities (for any character):
You must pause the game, then hold L and press X, Y, A,
Left, Left, Right, Right.

Never get hit (by items):
You must pause the game, then hold L and press X, B, B,
Left, Left, Y, B.

Have fruits (during entire race):
You must pause the game, then hold R and press B, X, 
Left, Left, Y.

50 balls:
You must pause the game, then hold L and press Up, X,
B, Y, Left, Left.