World 9-1: Rainbow Run
Look for a pair of bricks above the first set of rainbow rollers. Use a Cat Suit to scramble up the wall here and find a warp pipe. This leads to a secret area, where you have to manipulate switch panels to roll two balls to pressure pads. Do this to earn a star.
Stamp: Bowser Flag
You can grab this while riding Plessie. It's among some gold rings on the right side of the track.
Immediately after riding Plessie, you'll disembark near two unlit torches. Hit the invisible block on the left side of the platform for a Fire Flower, then light the torches to reveal a cloud that blasts you to a secret area. Step on all the colored blocks here before the music times out to create an eight-bit Link, then claim your star prize.
Also after riding Plessie, you'll hop off near a hollow green star. Hit it to spawn eight green coins; collect them in time to receive a star.
World 9-2: Super Galaxy
You'll spot the first star above a brick along the path. Jump onto the brick, then jump up again to nab the star.
Just past the checkpoint, you'll encounter a trio of Octoombas. Use blocks to reach this high platform, then use a Cat Suit to scramble up the blue tiles to the stamp.
When you reach a checkerboard of the red and blue squares, look to the west edge of the section. A red square here holds a star.
This is found underneath the warp box the end of the level. You'll have to run out and grab the star, then get back before you get spun into space.
World 9-3: Rolling Ride Run
Not too far into the level, you'll roll past a mysterious box. Hop inside to reach a hidden area. Here, you have ten seconds to run through the inside of a roller like a hamster wheel to reach a star. You can do it!
You'll find this on the series of rollers past the checkpoint. Hop to the little roller before the next platform; rotate it to bring the star to you.
Stamp: Tiny Rosalina
When rolling the giant roller near the end of the level, look south for a little pathway. Down there is a Rosalina switch; use her to hit it and spawn a stamp.
You usually have to play cooperatively to get this one. Further toward the end of the stage is a little alleyway with some bees. At its end is a two-man elevator which will take you to the last star. You can also wall jump up to the ledges on the sides, then use a Cat Suit to scramble up the star.
Level 9-4: The Great Goal Pole
This one is hard to miss it's right along the path while you chase down the flagpole.
Stamp: Tanooki Mario
This too is easy to spot along the path, but it's guarded by a Walleye. You'll probably take a hit while grabbing it.
Again, this is easy to spot. Make sure to get back before the platforms fall too far.
This is found during the final stretch, when you actually have to catch the flagpole. Just before the straightaway of falling platforms, there is a series of them that ascend. Take them up to find this star above where the flagpole runs.
Stamp: Cat Suit Mario 2
Enter this from the map to claim another stamp inside.
World 9-5: Super Block Land
Before exiting the first area, hit these question mark blocks to create a tower that takes you to the tall platform here. Hop into the mystery box here to reach a hidden area. Inside, you have ten seconds to hit more question mark blocks, generating more blocks that let you reach the star. We believe in you!
Stamp: Tanooki Luigi
This is in a corner of the course right by the warp box. Fun fact: if you're regular size, you can ground pound through these big blocks by holding the move for a few seconds. Do this to bust through and find the stamp underneath.
You'll find this inside a grey block just past the checkpoint. Use a Bob-omb or loose bomb to blow it up.
Before taking the unlocked warp block out of the level, grab a Bob-omb from the Bowser Box on the right side of the course. Toss it into the big block above, then head up to that platform. Hit the question mark block here a few times to create a tower of blocks, then wall jump you way up them to find a star.