If you wander into the hills up north (beyond the big waterfall gap) late at night, you can see several bright stars close to the ground that make eerie noises. Barking at them turns you into a ghost for a few minutes and grants you the hidden "Late Night Conversation" achievement. (You can see a more detailed guide by Bō / Toni here.)
Unlike flowers or crystals, the stars appear at exact same coordinates every night. The following map is the results of my feeble attempt to catalogue all of them.

Based on the Meadow map by Nemo. The faint marks are the star locations that I think I found, but I may have gotten lost or put them in the wrong place on the map :)

If anyone would like to double check my findings it would be greatly appreciated :) Especially if you are an eagle with good graphics settings that can get a nice overview of the starry area.
Also, if you have anything else you'd like to see in this guide, feel free to suggest it because I've no idea how to write a proper guide.
Thank you for your attention, and good luck stargazing ;)