Get All Worlds:
Finish Challenge 18 and find the Hidden Key

Get All Cars:
Finish Challenge 8 and hit 11 Bonus Stunt Items

Get All items:
Finish Challenge 7 and hit 30 Barrels

Infinite Health:
Finish Challenge 17 and hit 179 Barrels

Infinite Turbo:
Finish Challenge 4 and get 8 Zappers

Infinite Ammo:
Finish Challenge 18 and hit 50 barrels

Timer Speed:
Finish Challenge 12 and hit 60 barrels