Unlockable: Watermelon Mania
Collect 9,999 Star Bits (the maximum amount) to convert all of the game's green coconuts into watermelons. This cannot be reversed -- hope you like watermelon!
After rescuing Luigi from the haunted mansion, he'll start helping you find power stars. The second star he finds is on Battlerock. Go rescue him and get the star.
Buoy Base Galaxy:
As soon as you get there, dive underwater and get the underwater missiles to hit the cage blocking the green pipe. Head down there, and you'll pop up on the bottom side of the plane. There will be missile launchers everywhere, so make sure you have full health and good jumping skills. The star will be inside. Get a missile to hit it and you get your second green star.
Dusty Dune Galaxy:
On the second level of Dusty Dune, where you start, right underneath is a Hungry Luma. Feed him the ammount of star bits he wants, a new planet will form and a launch star will shoot you there. Now, you must collect all 5 silver stars. This part is like an elevator thing; you land on the button and you go up or down go in every direction you can until you have all 5 silver stars.
Hint: Super Stomp
If you want to do a specialized stomp with Mario or Luigi, try the following. When you jump, before you press the Z button to do the Ground Pound, spin by shaking the Wii controller. As you are spinning, you can then press the Z button. The result? A stronger stomp that will actually track the enemy and follow it, kind of like a heat-seeking missile!
Hint: Infinite 1-Ups
If you grab any 1-Up Mushroom at the Comet Observatory, you can then head into any dome, back out, and arrive back at the Comet Observatory without having played any stage. This will make the 1-Up Mushroom you acquired reappear.
Hint: Enemy Sacrifice
This may be an obvious hint, but it's one worth stating nonetheless. When you fight most enemies, you can control what comes from their dead bodies when defeated (a sort of "sacrifice"). By jumping on an enemy, which is usually the more difficult way to kill most foes, a Coin will appear. Spin-killing or using other methods will usually result in a few Star Bits.
Hint: Extra Lives
Because you start each game with only four lives (after shutting the game off and turning it back on), you might be in need of some extra lives upon rebooting the game. You can usually get five right off the bat from Toad (he'll run around with a letter above his head, symbolizing a letter from either Luigi, or more likely Peach). There are four more strewn around the Observatory that you can acquire, however:
1.) On the moving green platform below the core.
2.) Under the stairs leading to the kitchen.
3.) Right at the top of the observatory. (Once the Red Star is unlocked)
4.) Underneath the entrance to the gateway to the stars. (Once the Red Star is unlocked)
5.) Next to the StarShroom (in a create)
Unlockable: Death Count
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 the final Star is acquired with both characters in this galaxy, you will be able to see your death count on your save file. This will allow you to see how many times you died while playing the game with both Mario and Luigi.
Unlockable: 121st Star
There is actually 121 Stars in the game and not 120. To get it, you must first have all 120 Stars. This unlocks Luigi. Beat the entire game with Luigi, defeating Bowser at the end. After this, a new galaxy will be unlocked. Finish this final galaxy to get the 121st Star.
Here's how to get Luigi. First, get all 120 Stars in the game. Then, return to Rosalina (on the Comet Observatory) and request to fight Bowser one last time. Defeat him and let the credits roll. Thereafter, a message will appear telling you Luigi is a playable character.
Hint: Second Player
If you have a second Wii controller, you can have a friend play along with you. While there won't be a second player on-screen, you will have a second cursor to use. This means that Coins, Star Bits, and the like can be collected by the second player. There's also some functionality in controlling Mario's jumps.
(Edited by Mods (clean and tidy up)