It seems NiGHTS has set a trend. To view some date-specific artwork, set your Saturn's date to 1/1, 2/14, 3/3, 5/5, 12/25 or the 24th of any month. Once you've done that, start the game and go to the Options screen. Once you view a new piece of artwork it will be saved to memory.
To turn all the characters into elephants, highlight the elephant and hold Start until the other characters change.
To play against Carbuncle, possibly the toughest opponent in the whole game, complete HARD mode (Schezo's story) without continuing.