Sunshine Seaside will introduce a new thing, called Key Coins. Basically, there is a Warp Box in the level, but it is initially locked, so you will need to collect five Key Coins in order to unlock it. At the beginning of the level, there is a dock near your starting position, which has some Biddybuds marching back and forth on it. There is also Key Coin #1 here, so navigate past the enemies to grab it. Continue right into the small pit with a bunch of ?-Blocks, one of which has a power-up (Boomerang Flower, if big). There are also three Goomba sand statues here, all of which you can break and with the right one actually yielding Green Star #1.
To the right of there will be a clear pipe, so go through it and out the top portion. Grab Key Coin #2 here, then head out towards the water. There is a little sand dune here and if you fumble around by the water, you should find a hidden tunnel that will net you Key Coin #3. Note that if you go towards the camera on top of the sand, you should see the Key Coin, so it’s not completely hidden. To the right of that is some more clear pipes and if you head downward, you will find a 1-Up Mushroom. There’s also a Red Ring here and upon collecting all eight Red Coins, you will receive a Boomerang Flower.
Once you’re finished with those, climb the nearby tree to find Key Coin #4, then head out to the island with the locked Warp Box. Just northwest of it will be a hole in the water, which is hiding a green pipe. Enter that pipe and inside, you will need to light up all of the tiles on the floor by simply stepping on them. You can also use the Koopa shells, if you want, but the space is pretty small and there’s a high chance of getting hit. Don’t forget the steps and once you get all of the tiles, you will spawn Green Star #2. Exit the bonus room and return to the main piece of land.
Continue right of the last Key Ring you got to find a bunch of Chargin’ Chucks gathered together. Hit them to break them apart to find Key Ring #5. This should unlock the Warp Box, but don’t use it just yet. Instead, to the right of where you encountered the Chargin’ Chucks is a hole in the wall. Jump through it to find Captain Toad being harassed by some Biddybuds. Defeat them all to have Captain Toad reward you with Green Star #3. Go ahead and take the Warp Box now, setting you on an island with the Checkpoint Flag.
There’s nothing on this island, so just use the pipe to be plopped down right on Plessie. The section here is nothing more than just gliding down a waterway, with some gaps. Just remember to keep an eye out for the two gaps back to back, as there will be some platforms following that on the right side. Jump on the platforms and on the final one will be the Stamp. Back on the main course, the end is right there, so go down the pipe to appear at the Goal Pole.