Enter these case-sensitve cheats to activate them:

johnnye - Unlocks all costumes for all riders.
universe - Unlocks all levels and events.
fleximan - Unlocks all special stunt slots. seeall - Unlocks all posters and photo slots.
watchall - Unlocks all videos.
hearall - Unlocks all songs.
sugardaddy - Gives the user $1,000,000.
dudemaster - Unlocks all riders (and their bikes).
garageking - Unlocks all bike parts.