I want more!
To go on with the main quest, it is recommended to get as many masks, bottles and Pieces of Heart as possible. Here is how you can get more:
Go northwest from the Music Box House and enter the Spirit House. Speak with the mystery man and give him 30 rupees to perform the ghost hunting. Use your sword to beat the first Poe Sister, then do the same with the yellow and blue ones. Finally, stand in the middle of the room and fire arrows at the last ghost's four clones to win a Piece of Heart.
The second day at night, go back to the Ikana graveyard. Talk to the walking Stalchildren wearing your Captain's Hat and get down under the grave. Use your Lens of Truth to find a hidden door to the north and open it. Then use the lens again to see an invisible path, as well as three Skulltulas and a few bats. Follow the path, then place a bomb next to the cracked wall and open the door behind it. Defeat the Iron Knuckle like the previous one to get another Piece of Heart.
Still in the graveyard, but during the third day at night, speak with the walking Stalchildren wearing the Captain's Hat and get in the grave. Take off your mask and talk to Igor who is looking for the Royal Family's treasure. Target him and slowly lead him to a pile of sand to make him dig it up. You have to find three blue flames, one of them at least being on the upper level. To get there, there are two yellow-green slab-lifts in the northeast and northwest corners. Bring Igor to one of them. For the northeast one, climb blocks nearby and for the northwest one, climb the ladder to the west. When all three flames are found, fire five arrows at the ghost when it's not spinning and be rewarded with a Piece of Heart (3DS version) / a bottle (N64 version).
All five Business Scrubs make you each win a Piece of Heart and a yellow rupee (200) in the end. Here is the full walkthrough from the beginning: Go to the Astral Observatory to get another Moon's Tear, then exchange it with the Business Scrub in South Clock Town for a Town Title Deed. Dive in his flower to reach the Piece of Heart lying on the Clock Tower's balcony. Head to the Swamp, climb the ladder leading to the Tourist Centre and as Link, trade your deed for a Swamp Title Deed with the Business Scrub. Use the flower and pick up the Piece of Heart located on the house's roof. Then go to the Goron Village, to the south part, and as Deku Link talk to the Business Scrub. Trade the Swamp Title Deed for a Mountain Title Deed, then use his flower to take the Piece of Heart lying on the ledge a little bit higher. Next, go to the Zora Hall and as Zora Link talk to the Zora in front of the west room. As Goron Link, talk to the Business Scrub inside and exchange your deed for an Ocean Title Deed. Then use the flower to get the Piece of Heart which is waiting for you on the upper platform. Finally soar to the Ikana Canyon, climb down the cliff by jumping into the water and head to the far east of the area. Near Sakon's Hideout, talk to the Business Scrub as Zora Link and exchange your Ocean Title Deed for a huge yellow rupee! Then use the flower to cross the river and get the last and fifth Piece of Heart of the Title Deed quest!