All championship races
At the main menu, press Square, Triangle, Left(2), Circle(2), L, R(2), Triangle to unlock all championship races.

Profile Password Cheats
At the main menu, select Options. Enter Player Profile and go to Enter Cheat. Enter the following exactly like you see written:

Password -- Effect
$moneybags$ -- 1500 Credits.
TO LAZY -- All ATVs except G-Ride and Fury bike
TRICK IT OUT -- All bike parts
N0GAME -- All events (that's a ZERO not an O)
SMOG TEST -- All exhausts
All Access -- Everything but Fury bike.
+THREE -- Ravage Talon ATVs
Billboards -- All Music Videos.
Duds -- All Rider Gear.
Dubs -- All Rims.

Expert Levels
If you want the Expert levels you have to finish the game on Normal with a Gold cup and only Gold.

Unlock Ice Tournament
Complete the Amateur National on Expert Level with a Gold cup. No other cup will suffice.

Easy ATVs
Complete every course in training and depending on what cup/trophy you get, you will get a prize:

Gold = 3 Talon ATVs
Silver = 2 Talon ATVs
Bronze = 1 Talon ATV.