Cheats:

Novice Avalanche challenge:
Enter JAS3IKRR as a code.

Hard Avalanche challenge:
Enter 2AUNIKFS as a code.

Expert Avalanche challenge:
Enter EATFIKRM as a code.

Extreme Avalanche challenge:
Enter 9AVVIKNY as a code.

8-Bit Soul board:
Get nine Time Trial Coins as Ricky.

Backbone board:
Get nine Time Trial Coins as Tara.

Bad Cat board:
Get nine Time Trial Coins as Akari.

Broken Forest board:
Get five Time Trial Coins as Rob.

Cold Front board:
Get five Time Trial Coins as Kemen.

Conquest board:
Get nine Time Trial Coins as Rob.

Crime Scene board:
Get two Time Trial Coins as Tara.

DNA board:
Get five Time Trial Coins as Tara.

GCN board:
Get two Time Trial Coins as Rob.

Happiness board:
Get five Time Trial Coins as Akari.

Mind Invasion board:
Get five Time Trial Coins as Ricky.

Mr. Beake's board:
Win two trophies in gate challenge mode by besting the high score on two courses.

NST 1080 Avalanche board:
Win fifteen trophies in gate challenge mode by besting the high score on fifteen courses.

Old School board:
Win six trophies in gate challenge mode by besting the high score on six courses.

Once Bitten board:
Get two Time Trial Coins as Akari.

Power Painter board:
Win nine trophies in gate challenge mode by besting the high score on nine courses.

Pulse board:
Get nine Time Trial Coins as Kemen.

Scaretactix board:
Get two Time Trial Coins as Kemen.

Winterborn board:
Get two Time Trial Coins as Ricky.

Alternate boards:
Get eighteen Time Trail Coins to unlock alternate designs for the regular boards.

Alternate costumes:
Successfully complete the hard challenge to unlock the corresponding alternate costume for your current character.

Ricky: Frosty Winterball (snowman)
Akari: Crystal Hayami
Kemen: Titanium Vazques
Rob: Bones Haywood (pirate skeleton)
Tara: Mimi Le Moose (moose) 

Hint: Ghost character:
Press C-stick Down once or twice during game play and your player will turn translucent.

For Walkthroughs Try These Sites:

www.gamespot.com