Kingfin's Fearsome Waters
This is a fun optional boss battle. It's also against a giant shark that's a skellington which makes it quite cool. There are plenty of shells and air bubbles, as well as a health boost and a 1Up in a chest, so there's no excuse for failure really. It's not like your target is small. Just hit him four times with a shell to crack his bones and earn a star. The more you hit him, the angrier he gets, and he'll send smaller Mario-seeking sharkbots after you, but these are easily evaded.
Bowser Jr's Lava Reactor
King Kaliente's Spicy Return
That octopus is back, and this battle is much the same as your first encounter. But before you reach him, you need to lure a Bullet Bill along those winding paths to free the first launch star, then hurry across the sinking green platforms and wallop two coconuts back at the smaller octopi. You're then ready to face Kaliente one last time. As before, you're looking to knock back his coconut attack three times in a row to cause damage. Things are slightly harder this time, what with the sinking platforms, but if you find a spot with three close together you can hop from one to the other without too much trouble. The blue flame creatures shouldn't give you any real bother, so its really only Kaliente's increasingly frantic fireballs you need to keep an eye on. Send him back to the magma depths (how does an octopus live in lava anyway?) and the Grand Star is yours.
BONUS: Sand Spiral Galaxy
Feed the hungry Luma next to the Engine Room with 1000 star bits to open this galaxy.
Choosing A Favourite Snack
This is quick, but tricky. As soon as you start, watch out for the Magikoopa and his fireball. He'll shoot once, then zip away to the next section. Follow him with the launch star, and make your way to the safety of the stone ledges on either side. You've got a choice here - Boo Mario or Bee Mario? Either way, you need to make it through this hazard filled tunnel. Boo Mario isn't troubled by enemies, but must avoid the spotlights. Bee Mario can ignore the spotlights, but reaching safe places with his limited flying range is a challenge. Whichever you choose, get to the end and use the launch star. You're now faced with two giant concentric circles, with the star in the centre. Grab the invincibility power up, so you don't have to worry about those mines, and use the wider areas of the outer ring to jump to the inner ring. From there, leap to the core planet in the centre and pick up the star.
Special Red Power Star
Gateway's Purple Coins
With all Grand Stars collected, the pathway to The Garden opens up. Along the way, you can stop off at the Gate, which leads you back to where you started this whole ker-azy adventure, and earn a fun bonus. Enter the little cottage to start the Gateway level, and then make your way to the top of the castle. Talk to Rosalina, and then accept the Red Luma's challenge. Pick up the red star, and you can now fly! Give a spin while jumping to take flight, and hold down A to hover in mid-air and point yourself in the direction you want to go. Collect all 100 purple coins from the level and not only will you be able to use red stars to swoop around the observatory (that's the secret that the Red Luma on top of the Bedroom keeps blathering about, by the way) but when you complete the game you'll open up the Purple Comet coin-collecting missions for even more stars.
BONUS: Boo's Boneyard Galaxy
Feed the hungry Luma next to the Gate with 1200 star bits to open this galaxy.
Racing The Spooky Speedster
This is a straight forward race, in which Boo Mario takes on the Spooky Speedster. He's naturally faster than you, but you can slip through the crystal walls with a shimmy of the Wiimote and collect question coins to spawn blue fans to blow you into the lead.
NOTE: None of the Garden stages contain a Grand Star, so if you're only interested in beating the game you can skip these. If you want all 120 stars though, you need to finish these as well.