Unused songs - And a couple prototypes.
At the title screen, press Left (x2), Right, Left, Right (x2), Left, Right (x2), Left (x2), Right. This will open a menu where you can select your starting stage, and toggle invincibility (Cheat) by pressing the L and R buttons.
Disabled Button Code
Another method exists to access the above secret menu, but it was disabled in the final version of the game. Enter Pro Action Replay code 00831B00, then on the LJN logo screen press A + B + X + Y + Select. This was probably a shortcut used by the programmers during development.
Enter Pro Action Replay code 01DEF745 to view this unused company logo screen during bootup.