After you defeat Bowyer in the Forest Maze, you can optionally explore the Pipe Vault.
Explore the Pipe Vault
From the first room of the Pipe Vault, go northeast and jump over the gaps. If you hit a fireball, you will have to battle Sparkys and other enemies. If you fall in, you won't take damage, but you'll be sent back to the start of the room.
Northeast from the lava room, there is a Goomba walking around. You can either fight it or jump over it. After that, at the second-to-last step in the stairs, there is a Thwomp. If it hits you, you won't take damage, but you will be sent back to the bottom of the stairs, where a Goomba might attack. If you are still on the stairs when the Thwomp lands, you will also be knocked down the stairs. So be sure to go up the stairs quickly so you can reach the top before the Thwomp lands.
When you get past the Thwomp, there is a Goomba at the upper level, and a Piranha Plant in a pipe. You can fight the Piranha Plant or run past it (you can't go down the pipe that it is coming out of), then go down the pipe at the northeast end of the room.
There are several pipes here, and you can go down all of them except the third from the southwest. The second and fourth pipes both lead to the same area, which has a row of coins along the bottom. You can run and crouch to get the Frog Coin from the end. Then you can jump on the spring to go back to the previous room, but before you continue on, go back southwest and go back down the second pipe again. Just southwest of the floating platform, you can jump to reveal a yellow block. Jump on it to reach the floating platform. There are two hidden chests here, one above the fourth tile from the southwest, and the other four tiles northeast of that, to get Frog Coins. Then hit the chest at the end to get a Fire Flower. When you are done, you can either go down the pipe in the southwest of the room, or jump on the spring in the northeast of the room.
Back in the room with the row of Piranha Plant pipes, you can go down the fifth pipe to find a Goomba-stomping mini-game, similar to Whac-a-mole. Talk to the mole man to play. Jump on Goombas to get points. Gold Goombas are worth 3. But you lose a point if you jump on a Spikey. When you're ready to move on, jump on the spring in the northeast of the room.
In the room with the row of Piranha Plant pipes, go to the northeast end of the room and use the pipe there.
This room is straightforward: just jump over the gaps, get past the Goombas and Piranha Plant, and go down the pipe in the northeast end of the room.
This room is also straightforward, but try not to land on the Chompweeds. You will lose coins if you do. Get past the Chompweeds and Piranha Plants and go down the northeast pipe.
Jump on the floating yellow block to reach the chest above, which contains a Fire Flower. Then go northeast to find another yellow block. Try to jump on it without landing on the Chompweeds, then carefully jump to hit the chest repeatedly and get the coins from it, without falling onto the Chompweeds below. Afterward, go northeast and go down the pipe. You will arrive at Yo'ster Isle.