To unlock some mini-games (the ones from Moto GP and a couple others), go into options menu and watch the text (not video) credits. Scroll down until you get to the credits for the mini-games. When it appears onscreen, press A. To confirm correct code entry, the screen with the "Mini Games" title will have the "A" in "Games" change to resemble that found that controller button.
Unlock Challenges 62 through 66. Challenge 67 will be legends mode.
Unlock Tracks Faster
In the options menu, simply reduce the amount of laps required to complete a race.
Mug Shots Hint
When you are in the basement on Sheriden you will be able to see some mug shots on the right wall.
Floating bike mode
Edit the logo on a custom bike to read "kingpin".
Play as Alex Barros
Get 250,000 points in stunt mode to unlock Alex Barros.
Play as Carlos Chera
Get 100,000 points in stunt mode to unlock Carlos Chera.
Play as Max Biaggi
Get 500,000 points in stunt mode to unlock Max Biaggi.
Play as Tohru Ukawa
Get 750,000 points in stunt mode to unlock Tohru Ukawa.
Play as Valentino Rossi
Get over 1,000,000 points in stunt mode to unlock Valentino Rossi.
Get 325 championship points to unlock the "Edgy" option.
Pop Video option
Get 375 championship points to unlock the "Pop Video" option.
Get 300 championship points to unlock the "Saturate" option.
Get 275 championship points to unlock the "Turbo" option.
Successfully complete all the championship challenges to unlock the Sheridan track.
Season highlights FMV sequence
Win first place on any track to unlock that track's highlight's FMV sequence.
Unlocking characters easily
Start stunt mode and win the race under the pro difficulty setting. Burn out at the start until the counter reaches about 3,000 and do not go off the track (or you will get the clean section). Do this for about eight tracks to unlock all the characters. Note: You will need a bike with at least all stats at about halfway on each bar.