Press Down reapeaditly at SEGA logo to activate excellent mode.
Debug Menu
At the Sega logo, press: Up, Up, Down, Up, 2, 1, Right, Left. If done right, you'll hear a "Whoosh". Now, pick a new or saved game, and before starting, you'll be presented with the debug menu.
Higgins
In battle 18 (vs Solo), search the third tile in the nearest wall (from where you start)
Music Test
First you need to have a saved game with the game completed, or you can use the Debug Mode code and turn on the "Game Completed" option. Then, to access the Music Test, either start a new game, or pick a saved game, and before the game starts, press Up + Start.
Rush
In battle 21 (vs Barbara), search the statue near where you start.
Turbo Mode
At the Sega logo, press: Left, Right, 1, 2, Up, Down, Up, Up (if done right, you'll hear a "Whoosh"). Now everything in the game will flow faster (text, battles, movement).
Cheat...