To enable these codes enter them when you are in a park. If you have entered them correctly you will hear a chime as confirmation.

Unlock ALL Parks:
Press Square, Circle, Triangle, Square, Circle, Triangle, Square.

Unlock ALL Park Rides:
Press Square, Circle, Triangle, Square, Circle, Triangle, Triangle.

Complete Mission:
Press Square, Circle, Triangle, Square, Circle, Triangle, Circle.

Unlock $50.000:
Press Square, Circle, Triangle, Square, Circle, Triangle, X.

Enter the following sequences while in the themepark to either get 50 grand or unlock loads of stuff, read on...

Get $50,000
Square, Circle, Triangle, Square, Circle, Triangle, X

Unlock all Parks
Square, Circle, Triangle, Square, Circle, Triangle, Square

Unlock all Rides
Square, Circle, Triangle, Square, Circle, Triangle, Triangle