After Woodfall Temple
After Woodfall Temple, several new options are now open to you with the addition of your Hero's Bow and the poisonous waters cleared. There are a couple things you can get before you set off for the Snowhead and the second dungeon.
Here is a list of items you can obtain in Clock Town, The Southern Swamp and the greater Termina Field:
Heart Pieces
Southern Swamp
Heart Piece #12 - At the entrance to the Southern Swamp (where the path forks towards the shooting gallery and the swamp proper), look for a tall tree and climb the vines to the top to get the Heart Piece.
Heart Piece #13 - When you enter the Deku Palace, pass the guards as Deku Link and take a left into the Inner Gardens. Avoid the patroling guards line of sight as you make your way into the next area and go all the way to the back to find the Heart Piece.
Heart Piece #14 - After you play the Sonata of Awakening at the entrance to Woodfall Temple, continue using the Flower pads to fly around the temple and over to a large chest with a Heart Piece.
Heart Piece #15 - Obtain the Moon's Tear again and trade it for the Town Title Deed. Take the deed to the Business Deku Scrub in front of the Swamp Tourist Shop, and launch off the flower pad onto the roof to get a Heart Piece.
Heart Piece #16 - Return to the entrance to the Southern Swamp and take the other path towards the Swamp Shooting Gallery. Get over 2,180 points to score a Heart Piece. This means shooting down each and every Deku Scrub, Crow, and Wolfos, with at least 6 seconds to spare (2,120 points to clear, +10 points per second)
Heart Piece #17A - With Koume rescued and the poison swamp cleared by purifying the temple, Koume will let you play a target practice minigame. Beat the minigame to earn a Heart Piece.
Clock Town
Heart Piece #18 - With the Hero's Bow, enter the Shooting Gallery in East Clock Town and get a perfect score of 50 to earn a Heart Piece.
Heart Piece #19 - Visit Honey & Darling's minigame shop in East Clock Town each day and beat all 3 games to score a Heart Piece.
Termina Field
Heart Piece #20 - In Termina Field, there are four holes in the ground At the North, West, East, and South parts of the map. These holes lead you to a room with four of the Gossip Stones (the West hole is under a boulder). Drop down in each hole as Deku, Goron, or Zora Link and play their respective songs that opened their respective dungeons to change the color of each giant stone, use only one song with its respective mask as they all MUST have the same color.
Masks
Southern Swamp
Mask of Scents - After rescuing the Deku Princess following the cleansing of the Woodfall Temple, she will tell you that the Deku Butler has arranged a gift for you in a shrine just to the right of the Deku Palace upon leaving. Head inside and do your best to keep up with the Deku Butler as he flies through the labyrinth. For an easier time, make sure you have the Bunny Hood equipped to keep pace with him.
Magic
Southern Swamp
Increased Magic Meter / Charged Spin Attack - With all the Stray Fairies caught in the Woodfall Temple, return to the Great Fairy Fountain near the Temple to get your reward - a charged spin attack.
Gear
Clock Town
Big Arrow Quiver - Visit the Town Shooting Gallery in East Clock Town and get a high score of 40+ to get the upgrade.
Southern Swamp
Biggest Arrow Quiver - Visit the Swamp Shooting Gallery near the entrance to the Southern Swamp and get a high score of at least 2,120 (every target shot) to earn this upgrade.
Miscellaneous
Termina Field
Once you have access to the Magic Beans and a bottle of water, go to the entrance to the Southern Swamp and look for a large tree trunk with a chest on top. Use the beans on the soft earth near it, and feed it water to grow a levitating plant that will reach the chest with 20 rupees inside.
When you are all set, head off north to Snowhead and towards the Mountain Town