After using Sharpedo to break through the rocks in Route 15, you arrive in Route 16.
Explore Route 16
In the Route 16 Pokémon Center, talk to the girl on the left. If you have used the Pokémon Cafes a lot, she will give you a Lemonade.
The girl on the left will ask you to catch Mimikyu, then come back and show her your Pokédex.
To the right of the Pokémon Center, there is an Aether Foundation building. Inside, you will find Sina and Dexio. If you have collected at least 10 Cells and Cores combined, they will tell you about the Reassembly Unit. They say that Zygarde has 100 total Cells and Cores. Zygarde's Forme will change depending on how many Cells and Cores it is made of.
Check on the console of the Reassembly Unit. If you do not have a Zygarde in your party or Boxes, choose Assembly, then choose Using the Zygarde Cube only. The machine will create a Forme of Zygarde. The Formes that you can create are 10% Forme (requires 10 total Cells), the 50% Forme (requires 50 total Cells) and Complete form (requires all 100 Cells).
To add more Cells to Zygarde, put Zygarde in your party, then choose Assembly, then choose Using Zygarde and the Zygarde Cube. Then choose Zygarde from your party.
If you do Separation, it is like releasing Zygarde into the wild, but the Cells go back into the cube for you to create another Zygarde. The new Zygarde will not retain anything that the previous Zygarde had.
Outside, check on the right side of the Aether Foundation building to crawl behind it. Go downward to get TM81 X-Scissor. Then go up and check on the north end of the building to crawl back through.
Go southeast from here to find a PP Up.
In the grass, you can battle Scientist Reid. He has a level 32 Charjabug and a level 33 Muk. You get 1524 Pokédollars for winning.
After this, you can go north from the Pokémon Center to reach Ula'ula Meadow.
Go up along the wooden bridge. There is a Poké Finder spot to the right.
There is also a green sparkle for the Zygarde Cube here.
Up ahead, there are some stairs down to the flowers. There, you can battle Dancer Mireille. She has a level 34 Oricorio. You get 1360 Pokédollars for winning.
To the right of the Dancer, there is a Repeat Ball.
Go back up onto the wooden bridge. To the right, you can battle Office Worker Michelle. She has a level 33 Torkoal, a level 34 Leafeon, and a level 33 Seaking. You get 1360 Pokédollars for winning.
Go northwest, to where you can battle Lass Rylee. She has a level 33 Sneasel and a level 34 Komala. You get 816 Pokédollars for winning.
Go to the left from Rylee, and go through the grass at the end to get an X Sp. Def.
Go back to the wooden bridge, and go north, then down the stairs. Go to the left to find an Adrenaline Orb.
Go back onto the bridge and go north. After the bridge ends, go east and through a patch of grass. There is a green sparkle for the Zygarde Cube there.
Keep going east to find Lake of the Sunne/Moone. Go north into the ruins. In the upper-right corner, you find TM03 Psyshock. There is nothing else to do here, but you can keep going north if you want to see the rest of the ruins.
Go back to Ula'ula Meadow, then go west to find the northern path that leads to Route 17.