5730 HP during Field Trip battle:
Press Start to pause game play after entering battle mode. Press B(3), C, B(3), C, A(2), Start.

Opening song:
Press C at the title screen.

All stats 999:
Start a new game. Enter the following actions at the screen where you choose your name or nickname. Press Up(2) at the "Last Name" field. Press Down(2) at the "First Name" field. Press Left, Right, Left, Right, X at the "Nickname" field. All stats, including Stress, will be boosted to 999.

All stats 573:
Start a new game. At the screen where you choose your name or nickname, enter KO-NA-MI-MA-NN (Konami-man in Hiragana) as your Adana (nickname). Enter any value for a birth date, blood type, and other fields. All your character's stats, including Stress, will be boosted to 573.