Collect 9999 star bits in the hub, and all the coconuts in the game become watermelons.
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.
Grand Finale Galaxy and Star 121 for Mario and Luigi *SPOILERS*
To unlock the final star and galaxy, collect all 120 stars with Luigi and beat Bowser. Doing so unlocks the Grand Finale Galaxy, accessible from the green trial planet, 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.
Hidden Character *SPOILERS*
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.
Starting Boost When Racing Cosmic Mario
When the countdown timer starts as you are racing Cosmic Mario, hold forward, then as it hits 2, hold down on Z to crouch, and when the race starts hit A right away and you will turn yellow for a few seconds and get a really good boost that will put you way ahead of Cosmic Mario.