Go to the Options Screen, press and hold R2 and press Up, Up, Down, Down, Left, Right, Left, Right, Square, Circle, Square, Circle to enable the cheat screen. Now you can enter one of the following codes:
TopBird: stay On Jet Ski
Hobble: unlock Normal AI On Difficult Tracks
AllOutAI: get tough AI
AllChar: unlock All Characters
Festival: unlock All FMVs
IBelieve: race UFO In Time Trials
F18: race F18 In Time Trials
Passport: all levels
NOTE: all cheats are case-sensitive