Rival challenge mode:
Complete the game in championship mode as any character to unlock rival challenge mode.

Black Diamond track:
Win championship mode as any character to unlock the Black Diamond track.

Hint: Easy points for secret characters:
Find a large ramp to do tricks off. Do a trick, then turn around and do it again. You can take as long as needed to build the points. Then, finish the race to unlock the secret character. This can be done on any track that you still need points for a secret character.

Hint: Losing sleds in rival challenge mode:
Do not challenge a rival on a track that you cannot win. If you challenge a rival and lose to him or her, the sled you used will be lost and have to be unlocked in championship mode again.