It's time for a walk on a long stretch of beach. There are Biddybuds and Koopas here, as well as orange disks with a key symbol. These are Key Coins. Collect five of them to unlock a warp box to move on. Walk to the right and you'll see sand sculptures. Stand on the right Goomba sand sculpture and ground pound it for the first Green Star. There are clear pipes along the beach. Go up one of them to reach a cloud for a Key Coin. There's another under a sand hill near three Koopas. Go out into the water and you'll find a hole. There's a pipe down there that leads to a room with Koopas on a tile floor. Clear the tiles for the second Green Star. The pipe will take you near the locked warp box. You'll find a Key Coin in a nearby tree and a 1-up in a clear pipe. Ahead is a Red Coin Ring, which will give you a Boomerang Flower. Use the boomerang on the nearby Chargin' Chucks, who are guarding the last Key Coin. Alternatively, ground pound the right Goomba sand sculpture for a Goomba Mask, which will make all enemies ignore you. There's a Gold Shell in the Bowser sculpture, which creates coins. Go all the way to the right and you'll find a hole in the wall. Inside, Biddybuds are chasing Captain Toad. Defeat them all for the third Green Star.
Go through the warp box to the midpoint. There's a Moneybag in the area. Take the pipe underground, where Plessie is. There's a narrow waterway here and you must cross narrow passages and moving bridges. On the right side are some high planks with the stamp. Jump at the top of the slope to reach them. At the end is the pipe that leads to the flagpole, where there are Biddybuds and a Koopa.
The stamp is of a Koopa kicked out of its shell.