All levels:
Go to the options menu, go to "Change Name", and enter XOPENSEZ. 

Custom corvette mode:
Win all C3 career mode races. 

Unlock Everything:
Go to options and go to "change name" and put "xopensez".

Route 66 Checkpoint Mode:
Win all C5 career mode races to unlock this race mode.

Circuit Endurance Mode:
Win all the C4 career mode races to unlock this race mode.

Hot Lap Challenge Mode:
Win all the C2 career mode races to unlock this race mode.

Never lose a race:
When losing a race or getting into to a wreck, press Start. Select "Restart Race" 
and the race will begin again, not be count against you.