This level has a lot of Clear Pipes for you to travel through, and portions of them are blocked by spike balls that can cost you Power-Ups or lives.
Green Star #1
Reaching the first Green Star requires that you first pick up the Super Star inside the Clear Pipe, then quickly guide over to the Green Star before the Super Star wears out, as this is the only thing that can destroy the spike balls on either side of the Green Star.
If you destroy the three spike balls inside this Clear Pipe with the Fire Flower, you earn a 1-Up.
Green Star #2
You’ll have to time your entry into this Clear Pipe in between groups of Fuzzies to safely reach the second Green Star.
Stamp
After the Checkpoint Flag there’s a one-way Clear Pipe that can take you to the next section. However, it passes directly underneath the Stamp. You can run on top of the Clear Pipe and use a Spin Jump to reach the raised pipe section to claim the Stamp. A Cat Power-Up makes this much easier.
You need to use the Red POW Blocks here to wipe out the spikes in these Clear Pipes—it’s the only way to proceed. Mind the nearby Fuzzies while you’re getting this done!
If you’ve got Cat Power-Up, you can skip the Clear Pipes and head straight for the Warp Box.
Green Star #3
The climb up to the top of the mountain here leads to a Clear Pipe, blocked by spike balls, that feeds directly into a Cloud Cannon. Destroy the spikes with fireballs (you can fool the nearby Fire Piranha Plant into doing this if necessary), then get to the Cloud Cannon. As soon as you arrive in the bonus stage, you can pick up a potted Piranha Plant to help tear through all of the enemies on your way to the third Green Star. Just be careful not to fall off the clouds early!
Crouching on this hill will let you slide through all the Piranha Plants on the way down.
The last obstacle before the Goal Pole is a winding Clear Pipe, leading to a locked Warp Box. You must use the pipe to reach four of the five Key Coins you need, while the first of these Coins is helpfully provided at the start of this challenge. Watch the Fuzzies that inhabit the pipes!