After The First Three Days
With the Ocarina of Time safely back in your hands, and the Deku Curse lifted, there's several things you can do After The First Three Days before you set off for the Southern Swamp and the first dungeon.
Here is a list of items you can obtain in Clock Town and the greater Termina Field:
Heart Pieces
Clock Town
Heart Piece #1 - If you haven't already, switch to normal Link and climb up to where the entrance to the Clock Tower is. The Heart Piece if right in front of the entrance.
Heart Piece #2 - If you missed it during The First Three Days, trade the Moon's Tear for the Town Title Deed. Now wait until midnight and head into the Stock Pot Inn through the roof via the Flower Pad. Go into the bathroom under the stairs and give the strange hand in the toilet the Title Deed, and he'll reward you with a Heart Piece.
Heart Piece #3 - Now that you are back to normal, head to North Clock town and climb the slide structure (or the platforms next to it), and hop across the platforms to the tree with the Heart Piece.
Heart Piece #4 - As normal Link, head to the Swordmaster's Dojo in West Clock Town, and pay 10 Rupees to slice up the targets. To get a perfect score - and the Heart Piece, use the Z/L Target jump slash by pressing A.
Heart Piece #5 - Obtain Kamaro's Mask by searching Northern Termina Field at midnight on the first day. Find the man on one of the giant mushrooms and play the Song of healing for him. Return to the Rosa Sisters who dance at night in West Clock Town, and use the mask to dance for them. They will reward your dance moves with a Heart Piece.
Heart Piece #6 - Enter the Postman's residence between 12 PM and 6 PM on the first or second day. While resting, he'll offer to let you play a game where you start and stop a timer at exactly 10 seconds. If you have trouble completing the game, consider getting the Bunny Hood.
Heart Piece #7 - Use the Flower Pad near the Great Fairy's Fountain in North Clock Town to enter the fenced off area and jump into the hole. As Deku Link, you can play a different mini-game each day that involves floating to various platforms without hitting the ground. If you manage to complete three mini-games (one each day), you'll be rewarded with a Heart Piece.
Termina Field
Heart Piece #8 - Go towards the entrance to Milk Road in Termina and look for a group of butterflies among some tall grass. Fall into a hole near them and defeat the Peahat inside for a Heart Piece.
Heart Piece #9 - Travel towards the Mountain Village in Termina Field and look for a hole near the giant mushrooms. Drop down and defeat the two Dodongos to earn a Heart Piece. Bring bombs if you are having trouble.
Heart Piece #10 - Use the telescope in the Astral Observatory and look around for a Deku Business Scrub, and follow him until he lands into a hole near the Observatory. Follow him inside and he will offer to sell you a Heart Piece for 150. Decline and he will drop the price down to just 100 Rupees.
Masks
Clock Town
Blast Mask - Enter North Clock Town around midnight on the first day. A thief will try and steal a package from an old woman - intercept him and see that he drops it. She will give you the Blast Mask as a reward.
Kafei's Mask - Enter the Mayor's Office in East Clocktown and talk to Kafei's mother through the right door, who will give you the mask.
Great Fairy Mask - Rescue the Stray Fairy once more from either the Laundry Pool during the day, or in East Clock Town in the evening. Return to the Great Fairy as normal Link to get the mask.
Bremen Mask - Journey to the Laundry Pool at night and talk to the man playing the instrument. Listen to his tale and he will reward you with the mask.
Termina Field
Bunny Hood - Enter Romani Ranch on the Final Day, and head all the way towards the West end. Enter the door on the right and use the Bremen Mask to make all 10 chicks march behind you. You will recieve the Bunny Hood as a reward.
Kamaro's Mask - Go to Northern Termina Field around midnight on the first day. You'll find a man on one of the giant mushrooms - play the Song of healing for him to get the mask.
Gear
Clock Town
Bomb Bag - Purchase the Bomb Bag in the Bomb Shop in West Clock Town for 50 Rupees
Big Bomb Bag - Once you have saved the old lady from the thief at midnight on the first day, you will be able to buy a Bigger Bomb Bag at that Bomb Shop in West Clock Town for 90 Rupees.
Clock Town Map - If you haven't already, pop Tingle's balloon in North Clock Town and speak to him to buy a map of Clock Town for 5 Rupees.
Adult's Wallet - Deposit at least 200 Rupees to the Banker in West Clock Town (South in the 3DS Version) and you will receive the Adult's Wallet.
Termina Field
Romani Ranch Map - Heading down Milk Road, shoot Tingle out of the air and purchase the map for 20 Rupees.
Miscellaneous
Clock Town
The following three chests can be opened again each time the 3-day cycle is reset.
With either bombs or the Blast Mask, return to the Bomber's Secret Hideout and go left once you reach the water. Past a Skulltula is a dented wall that you can blow up to reveal a chest containing a Silver Rupee worth 100 Rupees.
Go up to the ramps to the roof over the Milk Bar in East Clock Town where the Bomber with the Cuckoo was. Jump across the gap over the East Gate (you can roll off the edge of the roof to jump a little further), then jump across the window frames over Honey & Darling's Shop to the roof on the other side. Continue around the roof behind the Town Shooting Gallery sign to find a chest containing a Silver Rupee.
Check in to the Stock Pot Inn and go up to your room (It's the Middle Door in the upstairs hallway). Inside is a chest containing another Silver Rupee.
Termina Field
There is a chest in the far north corner of the entrance to the Southern Swamp, search the tall grass and clear the Deku Babas to reach the chest and get 20 Rupees.
Close to the chest is a hole in the ground that houses 3 more Deku Baba and another chest with 20 Rupees inside.
When you are all set, head off to the Southern Swamp and towards the Deku Palace