After you complete the trial on Mount Hokulani, you should go to Route 11 on your way to the next trial.
Explore Route 11
The girl near the entrance of the route will heal up your pokémon if they need it.
South of that girl, there is a hidden Stardust in the grass.
Go southwest to find a Big Mushroom in the grass, to the left of the Route 11 sign.
To the east, you can battle Athletic Siblings Alyssa and Sho. They have a level 27 Herdier and a level 27 Fletchinder. You get 1512 Pokédollars for winning.
Southeast of the siblings, there is a hidden Stardust in the grass.
To the east, you find TM74 Gyro Ball between some bamboo.
North of where you found the TM, there is a hidden Star Piece to the left of some grass.
There is a Tiny Mushroom in the right side of that patch of grass.
East of that, you can battle Preschooler Hayden. He has a level 26 Stufful. You get 312 Pokédollars for winning.
To the east, you can battle Black Belt Clayton. He has a level 28 Hariyama. You get 896 Pokédollars for winning.
To the right, above the grass, there is a Quick Ball.
South of that, there is a green sparkle for the Zygarde Cube.
South of that, there is a patch of grass that has a Big Mushroom in the right side of the grass.
Go farther east and the girl will open the captain's barricade for you if you have Electrium Z. She says that the next trial is the Ghost-type trial. Ghost pokémon are weak to Dark and Ghost moves.