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 entrance to Route 11 is south of Malie City.
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 have a double battle Athletic Siblings Alyssa and Sho. They have a level 28 Lilligant and a level 28 Whimsicott.
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.
South of that, you can battle Preschooler Hayden. He has a level 28 Dedenne.
To the east, you can battle Black Belt Alvaro. He has a level 30 Bewear. After the battle, Alvaro gives you TM66 Payback.
To the east of that, you can battle Black Belt Clayton. He has a level 30 Hariyama. You get 896 Pokédollars for winning.
To the east, above the grass, there is a Quick Ball.
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.