There's a mountain climb ahead of you, but first, climb up the tree to the left. You'll find a lookout Sprixie and the first Green Star. Drop back down and start climbing the mountain. You can get the Super Bell here. There are many POW Blocks on the lower portion of the mountain. Hit them with good timing (some of them move) to set off chain reactions that allow you to beat more enemies and access new areas. One of the POW Blocks is set into the wall. Hit the POW Block near it to reveal a pipe hidden behind it. The pipe leads to a P-Switch that makes three sets of Silver Coins. Collect them all for the second Green Star.
Return to the mountain and you'll find the stamp in a nearby area where you can wall jump. From this point forward, many of the platforms are clouds. Some clouds are invisible, so either touch the touch screen or blow into the microphone to make them slightly more visible. There's an invisible cloud near a tree on the left. Climb up the tree to reach a cat gear. Spin the gear with your claws, then climb up the tower to reach a cloud cannon. This will blast you to a special stage.
Grab the Starman from the item block and start running. Run in a straight line to collect the coins, beat the enemies for a 1-up, and stay on the boost pads. At the end is the third Green Star. Drop back down to the mountain and you'll see a giant Piranha Plant at the end. Hit it twice to defeat it and reveal a warp block. This will warp you near the flagpole.
The stamp is of Cat Goomba.