After getting the Forest Medallion, your next destination is Goron City. However, there are a few things that you can do first.
Learn the Prelude of Light
Return to the Temple of Time. If you are outside, you can save and reset to get there quickly. Talk to Sheik, who is standing by the Pedestal of Time. If you are playing this game on the 3DS, Sheik will mention that you can fight past bosses in your dreams by sleeping in your bed in the Kokiri Forest.
Sheik will teach you the Prelude of Light.
Get a Gold Skulltula
Go to where Hyrule Castle was to find a gold skulltula behind some rubble near the edge of the lava pit.
Get a Bigger Quiver
Go to Kakariko Village and play the target practice game in the new house in front of the windmill. If you get a perfect score, you win a bigger quiver. The rupees appear in the same places as in the target practice game when you were a kid, but they are in random order this time.
Hunt Big Poes
Now that you have the bow and arrows, you can go poe hunting. There are ten Big Poes that haunt Hyrule Field when Link is an adult. If you get rid of them all, you get a bottle. Find out the Big Poe Locations.
Get a Free Goron Tunic
Go to Goron City. There is one goron rolling around on a lower floor. You have to stop him by making a bomb explode on him. An easy way is to stand on the fence opposite the bomb flowers on the floor, then when the Goron approaches, shoot the bomb flower to make it explode before he passes.
Talk to him. Ask about the dragon, then ask about the Gorons. He will give you the Goron Tunic. Equip it.
Get a Gold Skulltula in Goron City
There is a gold skulltula on the small platform hanging from the ropes on the top floor. Stand on one of the ropes and use the hookshot to get the token.
Death Mountain Crater
Go into Darunia's room and pull the statue out from the wall. With the Goron Tunic equipped, go through the passage.
Stand between the two poles with the broken bridge, to the left of where you came from Darunia's room. Use the hookshot on the distant wooden post above the bridge to reach the other part of the bridge. Walk forward and Sheik appears, teaching you the Bolero of Fire. Now you can enter the Fire Temple.