Collect the hidden Rockstar barrels to unlock the corresponding cheat option. Enabling a cheat will stop your progress in Career mode and disable the earning of money and reputation from races.

Unlimited Nitro: 10 barrels
Never Damage Out: 20 barrels
Unlimited Special Abilities: 30 barrels
Top Down View: 40 barrels
No Police: 50 barrels
No Fines: 60 barrels