After Lana adds Lapras to your Ride Pager, you can return to Melemele Island and explore Kala'e Bay.
How to Reach Kala'e Bay
Kala'e Bay can only be reached from Melemele Meadow. In Melemele Meadow, go southwest through the flowers to find a hole in the wall. Go into the hole, then go south.
Go southwest to some water and Surf across it to find TM79 Frost Breath. Then go south and go down a series of one-way ledges. From there, go northeast until you reach some more one-way ledges. Keep following the path to find a cave exit. Go through it to reach Kala'e Bay.
Explore the Bay
You can get TM05 Roar on the large, partly submerged island in the middle, and there is a hidden Big Pearl on the south end of this island. There is a hidden Star Piece on the north end of this island in the middle of the small pool.
On a small island in the northeast of the bay, you can get an item.
There is a Poké Finder spot on the east side of the bay.
There is a beach in the southwest part of the bay where you can get a hidden Pearl.
Go to the far south end of that beach to find a Prism Scale.