Memorial Hill leads you to Akala Outskirts, the last area before you reach the Ruins of Life.
Go east through the grass and you can battle Fisherman Vernon. He has a level 23 Staryu and a level 23 Gyarados. You get 736 Pokédollars for winning.
Go northeast from the fisherman to find TM28 Leech Life.
Go back to the path and go north. There is a Tiny Mushroom hidden in the grass up above.
To the left, you can battle Black Belt Kenji. He has a level 24 Hariyama. You get 768 Pokédollars for winning.
In the grass southwest of the Black Belt, there is a hidden Tiny Mushroom.
Southwest of that, there is a green sparkle for the Zygarde Cube.
In the grass northwest of Kenji, there is a hidden Big Mushroom.
Go north toward the Ruins of Life. Team Skull Admin Plumeria battles you. She has a level 25 Golbat and a level 26 Salandit. You get 3120 Pokédollars for winning.
After the battle, go north to enter the Ruins of Life. During a cut scene, Lillie will heal your pokémon.
Afterward, Olivia will arrive. You can battle her if you are ready, or you can say "Not yet" to prepare first.
Battle Island Kahuna Olivia
Island Kahuna Olivia has a level 26 Nosepass, a level 26 Boldore, and a level 27 Lycanroc. You get 4320 Pokédollars for winning.
Afterward, Olivia gives you Rockium Z, and you get a new stamp in your Trainer Passport, which causes all pokémon up to level 50 to obey you.
Olivia says that you should go to Hano Grand Resort by going east from the Dimensional Research Lab and crossing the bridge.
There is nothing to do in the Ruins of Life because there are large boulders blocking the way inside. You can come back later after you can push large boulders. For now, just fly to Heahea City.
Be sure to go into the Dimensional Research Lab and talk to Burnet to get the Reveal Glass.
Then go outside and go east to Hano Grand Resort. Before you go into the resort, you might want to visit Hano Beach to the east.