After Snowhead Temple
After Snowhead Temple, several new options are now open to you with the addition of the Goron Mask, Hero's Bow and the spring returning to the mountain. There's a lot more to do, including getting a new sword, your old horse back, and a few 3-day sidequests, before heading to the Great Bay and the third dungeon.
Here is a list of items you can obtain in Clock Town, Snowhead and the greater Termina Field:
Heart Pieces
Snowhead
Heart Piece #30 - Repeat the steps to obtain both the Title Deed and Swamp Title Deed, and then take the Swamp Title Deed to the Business Deku Scrub in front of the Goron town as Deku Link. Use the launcher to get to the platform.
Ikana Graveyard
Heart Piece #43 - On the second night, go to Ikana Graveyard and use the Captain's Hat to make the enemies open the grave. After going down you'll encounter lots of invisible enemies, use the lens of truth to be able to see them and the path. When you reach a cracked wall, use a bomb to break it and enter the area. Go through the door and encounter Iron Knuckle, he's a tough enemy so be careful. After defeating him a Golden Chest will appear, open it to get this Piece of Heart.
Heart Piece #27B (3DS Version Only) - On the final night, travel to the Ikana Graveyard and speak with the Stalchildren circling a gravestone while you wear the Captain's Hat. Have them open the tomb and drop down. Inside, Have Dampe follow you around and dig up each mound of earth to find three blue flames. One of them is located on an upper floor, so lead Dampe to one of the sinking platforms that will raise him up, and quickly guide him to the last dirt piles. When all three blue flames are lit, a Big Poe will appear. Kill it with arrows and you will be rewarded with a Piece of Heart.
Clock Town
Heart Piece #10 - Go into the Treasure Chest Shop in East Clock Town and talk to the lady at the counter while wearing the Goron Mask. She'll task you with navigating a maze to the treasure chest at the end. Get to the end and you'll receive a Heart Piece.
Masks
Snowhead
Don Gero's Mask - If you haven't already, look for the frozen waterfall when you first arrive in the the snowy Mountain Village. There will be a Goron trapped on top of the waterfall who can't get down. Head to the Goron Village Shrine and after playing the Goron Lullaby, light up a Deku Stick from the flaming torch, and quickly light all the other torches in the room (Bunny Hood helps). The chandelier will start spinning, so use the ramp as Goron Link to launch into the chandelier and break the pots. When a frozen Goron food drops out, carry it back to the waterfall and help out the starving Goron. he will give you Don Gero's mask as a reward.
Romani Ranch
Romani Mask - After saving the ranch from the ghost-like aliens on the first day (see below), speak to Cremia on the second day to find that she will be leaving the ranch at 6 to make a milk run. Join her at 6:00 PM and help guard her cart as she takes a detour through the Gorman Racetrack, as two familiar bandits try to break the milk carts. You'll have unlimited arrows, so keep up the rapid firing to knock the bandits back. If you can defend the cart, Cremia will reward you with the Romani Mask (subsequent attempts may even reward you with a hug).
Garo Mask - Enter the Gorman Brother's Racetrack while riding Epona and the brothers will challenge you to a race. You'll need to beat both of them to win, so hang on the inside of the track, and time your carrot consumption to leap over gates (never fully consume all your carrots or it will take longer to recharge. If you can beat the brothers, they will give you the Garo Mask and tell you to scram.
Ikana Graveyard
Stone Mask - Use Epona to jump the gates on the road to the Ikana Canyon. Just before the steep cliff that blocks your progression, use the Lens of Truth on a nearby circle of stones to find an injured soldier. Give him a red potion and he will reward you with the Stone Mask.
Captain's Hat - With Epona free, ride east from Clock Town into Ikana Canyon and jump the gates, then continue on the path towards the Ikana Graveyard. Travel through the area until you come upon a giant Skeleton blocking a road. Play the Sonata of Awakening, then chase him as he tries to get to the top of the hill. He'll try and block your path with walls of fire that can't be passed until a few Stalchidren are defeated. When you can, shoot him with a bow to stun him then follow up with your sword to force him into combat. When he surrenders, he will lower the flames guarding a chest that contains the Captain's Hat.
Magic
Snowhead
Double Magic Meter / Charged Spin Attack - With all the Stray Fairies rescued from Snowhead, look for the opening at the base of Snowhead to find the Great Fairy Fountain. Return the fairies to claim your reward an increased magic meter.
Ikana Graveyard
Song of Storms - On the first night, travel to the Ikana Graveyard with the Captain's Hat and speak to the Stalchildren guarding a tombstone. Tell them to break it open and drop down. Light the torches in the next area to open the path, and then get ready to fight an Iron Knuckle. Play is safe and bait out his attacks and counter with your own to defeat him. Once he is gone, you can learn the Song of Storms from the large tablet.
Gear
Snowhead
Biggest Bomb Bag - Approach the Business Deku Scrub outside the Goron Village Shrine, and talk to him as a Goron (Before giving him the Swamp Title Deed. He will sell you the biggest bomb bag for 200 rupees.
Bottle #3 - With the Snowhead Temple clear, return to the Goron Village and look for an opening that used to be covered in ice. The Biggoron inside will offer to sell you Powder Kegs if you can successfully destroy the boulder blocking the path to the Goron Racetrack. Carry the Powder Keg all the way back towards the path to the Mountain Village, and throw the keg up the slopes (avoiding enemies) to blow the path up. Now you can enter the Goron Races - where the first place prize is a bottle containing Gold Dust. The race can be tricky, as the other Gorons can ram you and knock you out of your charge, and you can easily fall off the track and get lost in the dust - luckily the race costs nothing to enter and you'll be prompted to restart if you fall too far behind.
Razor / Gilded Sword - With Snowhead cleared, the Mountain Village Smith will be able to upgrade your sword to the razor sword for just 100 Rupees. This will take an entire day to finish, and you won't be able to use your sword in the meantime. If you want the very best sword however, make sure you defeat Goht on the first day (you can always defeat him and rewind time to challenge him again at the start of the dungeon). When you get the Razor Sword, you will be told that you can increase the sword even further with Gold Dust. Get the Gold Dust reward from finishing first in the Goron Races, and ask to upgrade the sword once more. This will take one more full day, with the Gilded Sword becoming available on the final day.
Romani Ranch
Epona - With the ability to use Powder Kegs, go back to the first day and blow up the giant boulder blocking the road to Romani Ranch. Talk to the girl Romani and she will ask that you help ward off ghost-like aliens coming that night. If you agree, pass her test and shoot all the balloons. If you succeed, she will teach you Epona's song, letting you summon your horse at any time.
Empty Bottle #4 - Agree to help Romani fend off the ghost-like aliens from the farm, and return to the ranch at 2:30 AM when the enemies appear. You will need to fend them off until around 5:30 AM, as they make their way to the barn. You can either use Epona to ride around firing at them, run around with the Bunny Hood, or find a place to snipe from like the hill in front of Epona's stall or the roof using the ladder behind the house. You can also use the Song of Inverted Time to slow the enemies down, but it will take longer to ward them off. Make sure you buy the map of Milk Road to see them on your map, and keep a look out for which direction the dog looks when he barks to find the closest enemy to the barn. If you successfully defend the barn, you will be rewarded with a Bottle of Milk.
Miscellaneous
Ikana Canyon
When you are all set, head off south with Epona towards the Great Bay Coast