Adding a bit of detail on top of the other guide :
- go to the chogeko island, and find the entrance of the private beach.
- [option] - you can explore the beach and collect fruits. You can also jump / dive from the high rock on the left side of the beach. If you do so you will discover an hidden cavern and collect a shell that is needed for another achievement.
- wait until sun goes down, night fall and moon appear. It could take a few minutes (up to 10mins i would say).
- on the sand of the beach, a « Sun bath » option is available. Try it when moon is visible. No sun bath possible, Yu will propose something else !!!
Achievement is yours