Bonus Stages (Demo version)
     Instead  of  just  pressing [A]+[B] at the main menu of the demo,
press  and hold [1]+[2] when pressing [A]+[B] to access two additional
stages.

Hidden Character
     Get  all  120  stars. Return to Rosalina on the Comet Observatory
and request to fight Bowser again. Finish the credits and you will get
a message saying that Luigi is now playable.

Grand Finale Galaxy and Star 121
     To  unlock  the final star and galaxy, collect all 120 stars with
Luigi and beat Bowser. Doing so unlocks the Grand Finale Galaxy, which
is the Star Festival seen in the beginning of the game. Collecting the
100 purple coins in this galaxy gets you the final star.

Homing Attack
     Spin  in  midair,  and  then  immediately  press Z to do a homing
attack on the nearest enemy or breakable item. 

Death Count
     The  Grand  Finale Galaxy, which is the secret galaxy that can be
unlocked after 120 Stars are gathered with both Mario and Luigi, holds
another secret.

     Once  this  final Star from the stage is acquired with both Mario
and Luigi, you will be able to see your death count on your save file.
You  can  now  see how many times you died while playing the game with
both Mario and Luigi.