Hidden options:
Enter the options screen, highlight the "Exit" selection, and tap Left until the hidden options are displayed.

Fight as Dural:
Hold Left at the character selection screen until the countdown reaches "one". Dural will appear as a selection.

Extended character selection time:
Press Down, Up, Right, A + Left at the character selection screen for 99 seconds of time.

Alternate costumes:
Press Down + A, Up + A, Down + C, or Up + C at the character selection screen.

No damage:
Hold B while highlighting player one's life selection at the options screen until the message "No Damage" appears. Then, press Start.