Walkthrough: After defeating Hau and arriving in Iki Town, you will be informed that the Kahuna is presently not there. He has left to run a quick errand, leaving you to do nothing for now in Iki Town. With nothing left to do, Kukui will suggest heading up Mahalo Trail to see the ruins where Tapu Koko calls home.
Items:
Z-Power Ring - From Hala After Festival
Fighinium Z - From Hala After Grand Trial
Ride Pager - From Hala After Grand Trial
TM54 - From Professor Kukui After Grand Trial
Stardust - From Woman In House After Returning Letter Found On Kala'e Bay
Decidium Z (Post Game) - From Hala If Rowlet Not Chosen As Starter
Incinium Z (Post Game) - From Hala If Litten Not Chosen As Starter
Primarium (Post Game) - From Hala If Popplio Not Chosen As Starter
Post Game
Details: After defeating the Main Storyline, you can return to Iki Town to challenge Kahuna Hala. You will also find the Glaceon User here when doing the Eevee Quest.
Trainers:
Glaceon User Rae - Level 55 Glaceon
Kahuna Hala (All Level 63) - Hariyama, Machamp, Bewear, Poliwrath, Crabominable