You have 100 seconds to complete the level and you need Peach to get one of the items. Go back from the start, towards the screen. Hop over the fence to find the first Green Star on a cloud. Go forward and you'll find Microgoombas. There are spinning platforms with Piranha Plants, so be careful when jumping. You'll come across a fountain with coins. Nearby is a bunny with the second Green Star. He'll run around the fountain, so try running on the edge of the fountain, but not in the fountain. This make take a while.
Ahead are more spinning platforms with Piranha Plants. Go to the left to find a switch with Peach's symbol on it. Have Peach stand on it to make the stamp appear. Go forward and you'll see a cloud cannon on top of a short pillar. This will take you to the clouds. Get a Starman from the item block and run to the third Star Coin. You'll land near the flagpole.
The stamp is of small Peach.