Change engines:
Register over 1242.8 miles (2000 km) on the odometer on any car to unlock the ability to switch engines for that car. If you finish 100% of the game, you can do it on any car.

Change the weather:
Start a Time Attack, Free Run or Vs. Battle game and choose rain as your weather condition. While selecting the starting point, hold L2 to make it foggy, hold L1 to make it snow or hold R2 to make it storm.

Thanks to Revolution reader Barney Rubble!