Go to the code entry page and enter "unlockallthings" to unlock all the cars, tracks, mods, and arcade race events.

Also as a bonus, a new car should appear in the car lot, the Acura Integra.

This car will cost 0$ but if you buy it, you can directly sell it back for $7,200. A great way to make money!

But wait, there's more! If you've already completed the career event path and unlocked event cars, the first time you return from a race you will automatically be rewarded those cars again and you can sell them for about $300,000 total!

Enjoy!