After you get Lapras added to your Ride Pager, you can explore Kala'e Bay, which you can reach from Melemele Meadow and Seaward Cave.
How to Reach Kala'e Bay
Go to Melemele Meadow on Melemele Island. Go southwest through the flowers to find a hole in the left wall. Go into the hole, then go south until the path splits. From there, go east down a series of one-way ledges. Then follow the path to find a cave exit. Go out from there to reach Kala'e Bay.
Explore the Bay
You can get a Dive Ball from the large island in the middle, and there is a hidden Big Pearl on the rock south of there.
There is a Poké Finder spot on the right side of the bay.
There is a beach in the southern part of the bay where you can get a hidden Pearl.
On the south end of that beach, you will find find TM05 Roar.