Isle of Dead: Secret Cave
In the Isle of Dead in the bottom right corner of the map you will find a tree.
You will also see some stone squares on the floor under you. Each slab counts as one yard. Go West and count 13 squares along the way, stop on 13. Next, go seven spaces North using the squares to help you. Stop on the seventh and dig there. You will find a secret cave!
In there you can find a power gem and a few rupees behind the crack in the wall, so bomb the wall, get the cash and power gem, then exit. This time, go west until you find some stairs, avoiding boulders and keese as you go along.
Go up the stairs, then go east to a chest containing a Treasure Map. Then go east and shoot an arrow at the switch to open a door to the sacred graveyard. Enjoy the extremely small side-quest!