Features
Green Stars
Green Star 1: Wall-jump the sixth gate once it's rising to get the star.
Green Star 2: Pass the gates to the right of the Warp Box by jumping after running at full speed and get the star. Do the same way back.
Stamp
Next to the clear pipe after the checkpoint, there's a Cat Luigi stamp. Get it by either climbing the wall in Cat form or ground-pounding the cloud above.