Unlock the following bonuses by completing the corresponding challenge:

BONUS - CHALLENGE
Underworld Track - Complete Challenge 2.
Glacier Track - Complete Challenge 11.
Sewer Pipe Arena - Complete Challenge 5.
Crankshaft Docks - Complete Challenge 8.
Treadwell Plaza - Complete Challenge 14.
Underworld Arena - Complete Challenge 17.
Sho-Stopper vehicle - Complete Challenge 6.
Surf Crate vehicle - Complete Challenge 12.
Krazy 8s vehicle - Complete Challenge 18.
HW Prototype vehicle - Complete all missions in adventure mode.