Near the entrance of the cave is an Escape Rope. Follow the path southward to a small pond. Near the water surface is a Heal Ball.
Continue down the path to the southeast. You’ll find the path forks to the north and south. Take the path to the south to another fork in the road. At this second split, go east to a dead end to find a Never-Melt Ice. Give this to a Pokemon to boost the power of their ice-type attacks.
Return to the previous fork and go further south to reach a Super Potion. Follow this path south and east until you see a large boulder on the south side of the wall. Investigate this boulder to find a hidden Star Dust.
Exit out of the cave to the east to reach Kala’e Bay. The only thing here is a Net Ball. In the future, you’ll be able to fish and surf around this area. With that all out of the way, return to Melemele Meadow and Route 3 after finding Nebby.