The following bonuses can be unlocked during game play by doing well and earning points:

AI Vengeance: AI cars retaliate when hit.
Front Wheel Drive: Car has front wheel drive.
Hardcore Physics: Hardcore realism.
Horn Ball: Press Up to shoot a paintball.
Horn Mortar: Press Up to shoot an explosive mortar.
Insane Racecar: Press Up to jump.
Mini Cars: Smaller cars.
Monster Cars: High suspension.
Oh Dear: One car on track replaced by monstrous-sized crew member.
Super Draft: Faster draft speed.