Get all 120 stars and return to Rosalina on the Comet Observatory. Request to fight Bowser again. Go through the credits and you will get a message saying that Luigi is now playable.
Grand Finale Galaxy and Star 121:
Collect all 120 stars with Luigi and beat Bowser. This will unlock the Grand Finale Galaxy. Collect 100 purple coins in this galaxy to get the final star.
Spin in midair and immediately press Z to do a homing attack on the closest enemy.
Starting Boost When Racing Shadow Mario:
When the countdown timer starts hold forward. Then as it hits 2 hold down on Z to crouch. When the race starts hit A right away and you will turn yellow for a few seconds and get a really good boost.
In the Honeyhive Galaxy, there is a continuous stream of rocks with red centers rolling down the first hill in the galaxy. If you hit their red centers, the rocks will explode and around 10 star bits will come out. Keep doing this and collecting star bits and you will get many 1 Ups.
Even More 1-Ups:
To get easy unlimited 1ups, all you have to do is go to the kitchen.Then go back out of the kitchen and go under the kitchen stairs to find a 1up.Then go back into the kitchen and go back out again. You will find the 1up in the same spot as before. Keep repeating this and you will have lots and lots of lives.
Coconuts to Watermelons:
If you manage to collect 9999 star bits in the hub, then all the coconuts in the game will become watermelons.
Once this final Star from The Grand Finale Galaxy is acquired with both Mario and Luigi, you will be able to see your death count on your save file.
Thanks to Revolution Readers Nicholas Travieso and Boringman54!