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.
Gadgets for completing challenges:
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
Unlockables:
Unlock the following bonuses by successfully completing the corresponding challenges:
BONUS – HOW TO UNLOCK
Underworld Track – Successfully complete Challenge 2.
Glacier Track – Successfully complete Challenge 11.
Sewer Pipe Arena – Successfully complete Challenge 5.
Crankshaft Docks – Successfully complete Challenge 8.
Treadwell Plaza – Successfully complete Challenge 14.
Underworld Arena – Successfully complete Challenge 17.
Sho-Stopper vehicle – Successfully complete Challenge 6.
Surf Crate vehicle – Successfully complete Challenge 12.
Krazy 8s vehicle – Successfully complete Challenge 18.
HW Prototype vehicle – Successfully complete all missions in adventure mode.