Hardcore realism mode:
Press Up, Down, Left, Right, White, Up, Down at the main menu. Alternately, get a 100 point rating on a track to unlock the hardcore realism mode option for that track.

Shoot paintballs:
Select single race or head-to-head mode. Press Up, Down, Left, Right, White, Up(2) at the "Race Day" screen. Use your horn (press Up) to shoot a paintball. Note: This also works in two player mode. Alternately, select practice mode and press Up, Down, Left, Right, White, Down(2) to enable paintballs.

View credits:
Press Up, Down, Left, Right, White, Left(2) at the main menu.

High suspension:
Press Up, Down, Left, Right, White, Left, Right at the main menu.

Mini cars:
Press Up, Down, Left, Right, White, Down, Up at the main menu.

Wire frame cars:
Press Up, Down, Left, Right, White, Right, Left at the main menu.

Richard Petty:
Successfully complete all Heat challenges with a bronze or higher rating.

Hint: Pace car:
After coming out of the pits, press Brake. Reverse back into the pits and hit the pace car at about 100 mph. If you hit it hard enough, it will move. Steer it into any oncoming traffic.

Hint: Big air:
At Sears Point there is a hill with a huge Valvoline logo on it. Drive up the hill and you will fly into the air -- and your car may catch on fire.
Go past the first hill at Sears Point (the one that if you go too fast you will ramp and hit the wall) to the Valvoline jump, but do not ramp it. Turn around and go to the left side of the screen next to the wall and accelerate. Go forward until you ramp (this jump can hurt your car more than the Valvoline jump). You can do 720s, nose dives, half flips, etc.