Super Mario Galaxy 2
Yoshi Infinite Flutters
Jump holding A and before you finish Fluttering (While still holding A) Press Z, then quickly release A, then Press A again. Rinse and repeat.
Banker Toad's Items
The toad that banks your star bits will unlocks some items as you deposit more star bits. Those are not for Mario, he will simply use them himself.
Green Star Challenge
Collect all 120 stars and beat bowser and you will get a new cutscene with Rosalina reading about the green star to her "children".
Play as Luigi in Any Level
Defeat Bowser at Bowser's Galaxy Generator for the first time, then check the Mail Toad for a message from him. Look for the L on the Spaceship, enter it, and then you become Luigi.
Unlock Green Prankster Comets
After getting all 120 stars, beat Bowser in the final battle again. After the credits, a scene will play with Rosalina and after the cutscene, Green Prankster Comets will appear over each Galaxy.
Unlocking Rosalina save file icon
There is a save file icon that represents Rosalina's head which is unlocked by collecting all 120 Power Stars and defeating Bowser again.
Coconuts Turn Into Watermelons
Remember In The First Game Collecting 9999 Star Bits Turned All Coconuts Into Watermelons, Well It's Back Again