Pause the game and press T, T, X, X, O, S, O, S, D, and L. If this is done correctly, you will hear a sound. Repeating the code turns it off.
Pause the game and press U, U, D, D, L, R, L, R, X, and O. If this is done correctly, you will hear a sound. This code can be repeated at any time.
At the title screen press Sx5, Tx7, and Ox3.
Select to play the "Original" Parodious. Begin playing and shoot the entire first wave of enemies, then in the next wave shoot only the first enemy and wait for about 20 seconds without shooting anything else. Now, all of the enemies will dissappear, and you'll be on a hidden stage that looks like a subway station.