Successfully complete the Batmobile level to unlock the time attack and extra missions under the bonus option.
After completing certain levels, movies and pictures will be unlocked. This includes movie footage, interviews, and pictures.
Successfully complete the game to unlock the Caped Crusader, Dark Knight, and World's Greatest Detective costumes.
To defeat Ra's Al Goul at the end, make sure you defeat all of the ninjas that are around him; he will not die until all ninjas are dead. Also, when Ra's Al Goul is trying to attack you with the sword, it is best to use counter.