This is the area slightly north east of Liyue Harbor. There are 3 unlit torches in the area marked above. Simply light them all with any Pyro attacks and a hidden chest will appear.
Buried treasure near campsite
Area northeast of Liyue Harbor, in the marked location, there is a campsite with a hidden buried treasure near the edge.
Guyun Stone Forest
To get to the marked location, head to north of the Geo Hypostasis island, and activate the windmill with an anemo attack to create an upcurrent. Alternatively, you can make an ice bridge by freezing the water using Kaeya to get there.
There are 2 treasures in the island that you can obtain by digging for them. One is near the broken boat, and the other near the big rock.
If you haven't picked up the treasure from the heart shaped island further to the north, you can use this device to create a wind upcurrent as well. Activate it by placing down a Geo structure using characters like Geo main character, Zhongli, or Ningguang.
Heart Shaped Island
You need 2 players for this part. In the island north of the Geo Hypostasis, or east of Yaoguang Shoal, there is a small island with a heart.
You can get there easily by going to the island where the Geo Hypostasis is, there is a small windmill just north of the boss that you can use to create an upcurrent. Use an Anemo attack to activate it.
Glide all the way to the island and stand on the heart with your friend, and a luxurious chest will appear. If it doesn't, try it with different gender characters, or logging out and log back in, because it can be a little buggy.
Unmarked Side Quest 2
All the way down on the southeast area of Liyue region, in Guyun Stone Forest, there is also another ship that you can board to get a side quest. You can reach there by swimming there, or by gliding from the top of the mountain near the domain area.
In Qingce Village, there is a common chest that blends with the color of the waterwheel, making it easily missable.
Near the river area, close to a little boy npc that's running around, there is a pile of dirt that you can blow away using an Anemo attack. After it's blown away, you can then dig in that location to get a treasure chest.
Next to waterfall
South of Qingce Village, there is a small area with a waterfall. It has 2 places with some crystal chunks. The one in the bamboos has a treasure chest that you will have to dig to get.
Jueyun Karst and Qingyun Peak
The marked spot is in the "J" shown in the picture above. To the side of the bridge, there is a frog statue that you can interact. Investigate it to reveal a hidden chest.
North of the previous location in Jueyun Karst, there is a hidden chest that only appears when you are close. The spot is near the rock shown in the picture above.
West of the last location, this chest also only appears when you are close. This one is at the top of the rock.
Teleport to the Statue of the Seven - Geo in Qingyun Peak, then drop all the way down from behind the statue. At the ground floor, you can find 3 frog statues with a treasure that you can dig in the middle of them.
Hidden in bushes
From Taishan Mansion domain, head east until you go up a small set of stairs. There are some bushes there that you can destroy. A hidden chest will appear after you clear it out.
Between Lingju Pass and Mt Tianheng
This area is west of Liyue Harbor. In between Lingju Pass and Mt Tianhend, there is a burnable wood located in the spot marked above. Once you light it on fire, several Pyro slimes will show up. Once you clear all of them out, a hidden chest will appear.
From the teleport waypoint west of the Pyro flower boss, head south and go to the spot shown in the marked location. There is a buried statue, and you can dig near the legs area to find a hidden chest.
It is absolutely accessible! Just takes like an hour of climbing into a tiny hole until you finally glitch the game out enough to get in. The chest is very much still there. If you need a more detailed explanation of how to get in I'd look it up cause i dont have any pictures to show you unfortunately