Floating bike mode:
Edit the logo on a custom bike to read kingpin.

Mini-games:
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 on that controller button.

Valentino Rossi:
Get over 1,000,000 points in stunt mode.