AT the title screen, press "up, up, down, down, left, right, left, right" on the D-Pad and then press "B" and "A". This displays "COWABUNGA" when players clear the story mode on the "Normal" or "Hard" difficulty setting.
Race Mode and Battle Mode Bonus Races and Arenas
If you select the "input password" option from the mode selection screen and input the patterns RDLDSMD,MDSMSDM, or SRMLDDR, you will get bonus races in race mode. Input DDRSMSR,SMRDLML, or LMSLSRS and you will get bonus arenas for battle mode.