Select the "Extras" option at the main menu, then select "Cheat Codes". Enter one of the following codes:

MYPRECIOUS - All boards
FREEVISIT - All championship locations unlocked
GOINGDOWN - All Leaf Sliding locations unlocked
NICEPLACE - Art Gallery
DONTFALL - Bonus mission "Legendary Wave" in "Pen Gu North"
TOPFASHION - Customizations for all characters
TINYBUTSTRONG - Play as Arnold
SURPRISEGUEST - Play as Elliot
SLOWANDSTEADY - Play as Geek
IMTHEBEST - Play as Tank Evans
KOBAYASHI - Play as Tatsuhi Kobayashi
THELEGEND - Play as Zeke Topanga
WATCHAMOVIE - Video Gallery and Spen Gallery

These characters become available as stage points are achieved.

Code - Effect

Arnold - Earn at least 30,000 pts in Shiverpool 2

Big Z - Earn at least 100,000 pts in Legendary Wave

Elliot - Earn at least 40,000 pts in Pen Gu North 3

Geek - Earn at least 60,000 pts in Pen Gu South 4

Tank - Earn at least 40,000 pts in the Boneyards
Tatsuhi - Earn at least 15,000 pts in Pen Gu South 2

More Accessories - These only need to be unlocked once and they'll be available for every character.

Code - Effect

Back Accessories - In Shiverpool 1, obtain 30,000 pts
Wrist Accessories - In Pen Gu South 3, obtain 40,000 pts