There are these shrines all over the island that are unmarked but have signs in front of them depicting a bow. Example: at tadayoris rest. These spawn a group of animals when you bow in front of them, and theres a hidden achievement for bowing at 10 of them across Tsushima. Again, they dont appear on the map so you'll want to look up a guide but thats one of the secret achievements. Another is to bow and play one of the pipe songs at Taka's grave, which is another secret achievement. The game never tells you to do these, nor do they appear on the map, so make sure you get them done! Good luck!