From main menu go to Extras and then Cheat code and enter these codes and then press enter to apply.

Code - Effect

IMTHEBEST - New character - Tank Evans
KOBAYASHI - New character - Tatsuhi Kobayashi
THELEGEND - New character - Zeke Topanga
MYPRECIOUS - Unlocks all boards
GOINGDOWN - Unlocks all Leaf Sliding locations
NICEPLACE - Unlocks Art Gallery
TOPFASHION - Unlocks customizations for all characters
WATCHAMOVIE - Unlocks Video Gallery & Spen Gallery
DONTFALL - Enables Bonus Mission 'Legendary Wave' in 'Pen Gu North'
FREEVISIT - Gives access to every championship location
TINYBUTSTRONG - New character - Arnold
SURPRISEGUEST - New character - Elliot
SLOWANDSTEADY - New character - Geek