After you defeat Kahuna Hala to complete the Melemele Grand Trial, you can use Tauros to enter Ten Carat Hill.
Open Ten Carat Hill
Go to the Pokémon Research Lab south of your house, then go southwest to the rocks that block the entrance to Ten Carat Hill. Use Tauros to break the rocks and enter the cave.
Go upward from the entrance to find an X Attack.
Go downward from there and use Tauros to break through the rocks. Get off of Tauros and go through the narrow path up above to find a Super Potion. Keep going leftward along this ledge to find an Ultra Ball.
Jump down from the ledge, and use Tauros to go to the left through the rocks. When the path splits, break through the rocks on the left and go west up the long path. Before going outside, check on the rock to the left to find a hidden Protein.
Then go outside to get Flyinium Z. After you collect it, there will be a cutscene.
Then go back inside and go back to where the path splits, then go north to reach Farthest Hollow.
Go upward and there is a Hard Stone partially hidden behind the grass to the left. Go farther upward and you find a Nugget. Go left and down from there, and go through the grass to find a Great Ball.
Go to the northeast corner to find a Burn Heal.
From there, south to enter some grass. Go through the grass to reach a slope. Go up the slope to find TM23 Smack Down.
That is all that there is to be found in Ten Carat Hill, so you can either use an Escape Rope to go back to Route 1 Hau'oli Outskirts, or you can go back down the slope, back through the grass, and go left and down to exit the area.
After this, you can go to Big Wave Beach Mantine Surf to Akala Island.