Cheat mode:

Select "Options" at the main menu. Then, select the "Player Profile" option, then
choose "Enter Cheat". 

1,500 credits.:

Enter $moneybags$ as a case-sensitive code.

All bikes except for the Fury:

Enter All Access as a case-sensitive code.

All rider gear:

Enter Duds as a case-sensitive code.

All rims:

Enter Dubs as a case-sensitive code.

All exhaust pipes:

Enter Smog test as a case-sensitive code.

All music videos:

Enter Billboards as a case-sensitive code.

All championship races:

Press Square, Triangle, Left(2), Circle(2), L, R(2), Triangle at the main menu.

Training bonus:

Complete all courses in training mode with a "Gold" rank to get three Talon ATVs.
Complete them with a "Silver" rank to get two Talon ATVs. Complete them with
a "Bronze" rank to get one Talon ATV.

Expert levels:

Win a gold cup for a level under the normal difficulty setting.