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

Legends mode:
Unlock Challenges 62 through 66. Challenge 67 will be legends mode.

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.

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