You can enter codes when you are in the main menu, and press certain buttons. Try some of these:
Rock Meter Always Green Code
While in the main menu main quickly press Yellow, Blue, Orange X2, Blue, Blue, Yellow, Orange. Once you enter the code correctly you will know it by a message that appears briefly.
Air Guitar Code
While in the main menu main quickly press Orange X2, Blue, Yellow, Orange. Once you enter the code correctly you will know it by a message that appears briefly.
Monkey Code
While in the main menu main quickly press Blue, Orange, Yellow X3, Blue, Orange. Once you enter the code correctly you will know it by a message that appears briefly.