After you get Zora's Sapphire, there are some things that you can do to prepare for the next part of the game.
Get a Gold Skulltula
After getting Zora's Sapphire, turn it to night time in Zora's Fountain and get up onto the big log in the water in the southeast part of the fountain. There is a gold skulltula on the wall near the end of the log. Use your boomerang to get the token.
Learn Epona's Song
If you haven't learned Epona's Song, go to Lon Lon Ranch in the center of Hyrule Field and play your Ocarina near Malon to learn Epona's Song.
Plant Magic Beans
Plant a magic bean in the soft soil near the entrance to Dodongo's Cavern in Death Mountain Trail, and plant another bean inside of the Lost Woods near the entrance.
Create Scarecrow's Song
Go to Hyrule Castle Town
Go to Hyrule Castle Town. If it's day time, you will notice that the drawbridge is closed and the torches are lit, even though it's not night yet.
As you approach the drawbridge, the sky will go dark. Watch the cut scene when you reach the drawbridge.
After the cut scene, jump into the moat to the right of the drawbridge and dive down to get the item that was thrown into the water. You now have the Ocarina of Time. Watch the scene to learn the Song of Time.
Play Bombchu Bowling
Before you move on to the next part of the game, you can play bombchu bowling in Hyrule Castle Town. You can win a bigger bomb bag, a piece of heart, bombchus, and bombs.
Get Din's Fire
Go to Hyrule Castle. Climb the vine to get past the stone arch. Straight ahead from the arch, you see a path with a boulder at the end. Bomb it to reveal a great fairy fountain. Play Zelda's Lullaby while standing on the Triforce symbol to receive Din's Fire.
Collect Skulltulas and Pieces of Heart
Go to the Temple of Time
When you are ready to move on to the next part of the game, return to Hyrule Castle Town. In Hyrule Castle Town, there is an exit to the right of Happy Mask Shop. Use it and follow the path to reach the Temple of Time.