After you complete all of the temples, Rauru tells you to go to the Temple of Time. But before that, here is a list of the optional things that you can do to collect everything in the game.
Collecting Gold Skulltulas
Check the Gold Skulltula guide to find any that you might have missed. There are 100 in total.
Collecting Pieces of Heart
Read the list of pieces of heart to check if there are any that you might have missed. Link can have a total of 20 hearts in his health meter.
Read the upgrades page to find out how to increase your carrying capacity of bombs and other items, as well as increase your abilites.
When you are ready, play the Prelude of Light to go to the Temple of Time, or just go there on foot. There will be a cut scene.
Afterward, go to Ganon's Castle, where Hyrule Castle was in the past. After another cut scene, you can enter Ganon's Castle.