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.

Complete the following challenges to unlock the corresponding gadgets:

CHALLENGE - GADGET
Challenge 1 - Laser Cannon
Challenge 3 - Sonic Boom
Challenge 4 - Atom Blaster
Challenge 7 - Freon Bomb
Challenge 9 - Energy Shield
Challenge 10 - Jet Booster
Challenge 13 - Super Zapper
Challenge 15 - Magnet Mine
Challenge 16 - Doom Disks

Unlock the following bonuses by performing the corresponding tasks:

BONUS - HOW TO UNLOCK
All cars - Collect 11 Bonus Stunt items while completing Challenge 8. Note: The Bonus Stunt Items are the purple balls with silver rings rotating around them.
All worlds - Find the Hidden Key while completing Challenge 18. The yellow key is in the Mines, across a trestle at the end of a tunnel.
All gadgets - Intentionally hit 25 Barrels while completing Challenge 7.
Laser upgrade - Note: You must get an "A+" rank when doing the following. Complete all challenges, then unlock the "Timer Speed" cheat. Set it to 10% speed. Change the difficulty level to very hard then complete Challenge 18. Get the "A+" rank to unlock the "Laser Upgrade" cheat.
Unlimited ammunition - Intentionally hit 50 Barrels while completing Challenge 18.
Unlimited health - Intentionally hit 179 Barrels while completing Challenge 17.
Unlimited turbo - Collect 8 Zappers while completing Challenge 4.
Timer speed - Intentionally hit 13 Detour Signs while completing Challenge 15.