This is it - it’s time to meet up with Zelda and collect the Ocarina of Time so you can enter the Sacred Realm and get the Triforce! As you approach the castle, however, dark clouds begin to fill up the sky…
Now that you’ve got all the Spiritual Stones, head back to Hyrule Castle to meet Princess Zelda. When you arrive you’ll witness a plot twist - one that I won’t spoil for you here. Zelda has to leave you the Ocarina of Time, which you can find at the bottom of the Hyrule Castle moat. Jump into the water and dive down to grab the ocarina.
After you’ve retrieved the Ocarina of Time, Zelda will contact you telepathically to teach you the melody you need to open the Door of Time: the Song of Time.
Now all that’s left to do (aside from complete any of the optional sidequests now available to you) is enter the Temple of Time in Hyrule Castle Town. You can find the temple at the northeast road leading out of the market square. Head inside and approach the altar right in front of the great stone door at the far side of the main chamber and whip out your Ocarina of Time. Tap out the Song of Time and you’ll activate the power of the Spiritual Stones. The Door of Time will open, revealing… a sword?
This is the Master Sword, the blade of evil’s bane. Its ancient magic seals the entrance to the Sacred Realm. Fortunately, you can withdraw it from its resting place by approaching it and pressing the A button. Once you do, however, you’ll be enveloped in a bluish light…
The Boomerang doesn’t really open up too many new obtainables, aside from Skulltula Tokens, but you can complete the Happy Mask Trading Sequence now to round out all the sidequests you can do as a child. Speaking of Skulltula tokens, if you’ve been keeping up to date, you can also pick up the Giant’s Wallet at the House of Skulltula. It can hold up to 500 Rupees!
You can cap off the Happy Mask trading sequence now that you’ve gotten all three Spiritual Stones. Pick up the Bunny Hood from the Happy Mask shop if you haven’t already and head out to Hyrule Field. Follow the trails in a clockwise circuit around the field and you’ll eventually run into a strange jogger who wants to run like the wind. He’ll stop at intervals to sit down and rest, and it’s at this time that you should spring up on him and make your sale. His offer is extremely generous - he’ll fill up your entire wallet! (This is a really sweet deal if you have the Giant’s Wallet: see Optional Secrets above.)
Now just head back to the Happy Mask Shop and pay the salesman 50 Rupees for the mask and the entire collection of masks will be open to you, including four all-new ones!
Once you’ve completed the Mask Trading Sequence, you can also take the Mask of Truth to the Lost Woods’ “Forest Stage” and show it off to the Deku Scrub audience there to receive an upgrade to your maximum Deku Nut count. If you’ve already gotten the previous upgrade, this will bring your capacity up to 40.
Ocarina of Time screenshots courtesy of AceofSuns. Screenshots will eventually be replaced with 3DS version images, which are not presently available for hopefully obvious reasons.