Do the following achievements to enable the corresponding effect:
All Worlds: complete Challenge 18 and find the Hidden Key
All Cars: complete Challenge 8 and hit 11 Bonus Stunt Items
All items: complete Challenge 7 and hit 30 Barrels
Infinite Health: complete Challenge 17 and hit 179 Barrels
Infinite Turbo: complete Challenge 4 and get 8 Zappers
Infinite Ammo: complete Challenge 18 and hit 50 barrels
Timer Speed: complete Challenge 12 and hit 60 barrels