Super Mario Galaxy 2 (Wii)

Play as Luigi anytime
After you beat Bowser in World 6, you'll be able to switch between Mario and Luigi anytime on Starship Mario.
Unlock World S
Beat Bowser in World 6 to unlock the Special World.
Unlock Green Stars
Collect 120 Power Stars, then beat Bowser in World 6 again. After the new ending sequence, 120 green Prankster Comets will scatter throughout the seven worlds.
Unlock Grandmaster Galaxy
The final galaxy appears in World S when you collect all 120 Green Stars. "The Ultimate Test" in Grandmaster Galaxy has star #241 and Comet Medal #49.
Unlock "The Perfect Run"
Beat "The Ultimate Test" in Grandmaster Galaxy, collect its Comet Medal, and deposit 9,999 Star Bits at the Banktoad to unlock the Prankster Comet named "The Perfect Run" in Grandmaster Galaxy. If the Banktoad never had 9,999 Star Bits, "The Perfect Run" won't appear until you deposit them, even if Mario is holding 9,999 Star Bits.
Thanks: TYP ("The Perfect Run" requirements testing)