Super Mario Galaxy

# Play as Luigi

Collect all 120 Stars. Then, go to Rosalina on the Comet Observatory, and request to fight Bowser again. Defeat Bowser, and watch the credits. Once they finish, you will get a message saying Luigi is now playable.
# Grand Finale Galaxy

Collect all 120 Stars with Luigi, and defeat Bowser to unlock the Grand Finale Galaxy, which is the Star Festival seen in the beginning of the game.
# Star 121

Collect 100 Purple Coins in the Grand Finale Galaxy to get the final Star.
# Homing attack

Spin in mid-air, then immediately press Z to hit the nearest target.
# Assistant

While playing the game, use a second Wii-mote. Although a second player will not appear, the second Wii-mote will have its own pointer, allowing another player to assist by collecting items and helping Mario's jumps.
# Pokemon reference in Buoy Base Galaxy

In Buoy Base Galaxy, above the tower where you collect the Blue Star Pieces, is a little planetoid that is a sphere with two red panels on top and a lighter bottom with a screw in the middle. It looks exactly like a Pokeball.
# Pikmin reference in Space Junk Galaxy

One of the planets is shaped like Captain Olimar's rocket ship that appeared in Pikmin and Pikmin 2.
# Bonus stages (demo version)

At the main menu, hold 1 + 2 and press A + B to access two hidden stages.