The Legend of Zelda: Majora's Mask/Southern Swamp

As mentioned by Tatl, you need to head to the swamp.

To proceed, you need to head through the swamp area. Bypass the first building, and head to the river in the left; you should see a large cauldron. Head down the pathway behind it (past the Deku babas) and you should see the Magic Hag's potion shop. Bypass this as well, and head into the forest behind the building.

Speak with the monkey, and follow him. He will lead you to another hag that was injured by the skull kid. Talk to her and return to the potion shop to get a Red Health potion (along with a glass bottle). Head back into the forest and give the potion to the injured hag.

Once you leave the forest, you will now need to head to first swamp building. Head to the cruise department, and because you helped her in the woods, she will give you a free cruise. You will receive a pictograph box that can take pictures, and get to perform a swamp cruise to the Deku Palace.

Deku Palace
When you transform into a Deku Scrub, you will be allowed to enter into the palace to watch the public humiliation of the monkey for kidnapping the princess. He claims that she is in the shrine and will be eaten by a monster. After speaking with him and leaving the royal chamber, you will be told by a monkey that you need to plant a magic bean on the eastern side of the palace and reach the secret entrance.

First, you need to learn how to use them. Enter the eastern palace garden (on the right of the entrance) and sneak past two rooms of guards to reach a hole. Enter the hole to speak with a person selling magic beans. He will give a free sample and explain how they work: you need to plant them in soft soil, and either water them or have the weather water it for you. You can purchase extras for 10 rupees each, but make sure you leave with at least bean and fresh water.

Exit the palace, and head around to the east outside the palace. Plant a bean in the soft soil patch and water it. (on the second day, the weather will water it for you). Ride the plant into a side entrance, and transform into a Deku. You will need to go to the other side of the garden without falling down; go from flower to flower to the other side of the garden and enter the main palace.

Sonata of Awakening
When you enter the cage, you will need to speak with the monkey. He won't give much information initially, but when you switch to human form, he will ask if you have an instrument. Your ocarina isn't loud enough, but when you switch back to deku form and show the deku pipes, he will teach you the Song of Awakening:

This song allows you to access the woodland temple. Before you are carried off, the monkey tells you to take the shortcut to reach the temple.

Song of Soaring
When you are heading to the woodland temple, you will see an owl inform you about the Song of Soaring:

This song now allows you to use the stone owls as warp points, thus allowing quick travel to and from town in order to meet certain schedules. In addition, it allows you to warp to a dungeon entrance. However, it won't work in other indoor environments.

Swamp places of interest

 * South of Termina Field is a small path that forks off to a shooting range. You need to bring your bow do play.
 * South of Termina Field is a Piece of Heart on a dead tree. This tree is climbable, but there is a Bad Bat guarding it. If you don't have the bow yet, use Deku Link's bubbles to take it out so you can climb to it.
 * The swamp building has a Business Scrub outside of it that sells Magic Beans to Deku Link for 10 Rupees each. If you give him the Land Title Deed, he will give you the Swamp Title Deed and move to the flower in Clock Town, allowing Deku Link to use his old flower to get a Piece of Heart.
 * The western palace garden has a piece of heart.