Close to the Statue of The Seven in Mingyun Village, you can find a small hilichurl object that you can destroy. Some hilichurls will spawn 1 at a time after destroying them. Clearing them out will reward you with a chest.
North west of Mingyun Village, close to the beach area, there is a spot where you can burn the wood surrounding a circular platform. Some Pyro slimes will appear, and an Abyss Mage. Clear them out and a hidden chest will appear.
Burnable Wood 2
South west of the Statue of the Seven - Geo, in Mingyun Village, there is another spot with a pile of wood that you can burn. Once it's lit, a Pyro Abyss Mage will show up. Defeat him and a hidden chest will appear.
Cuijue Slope Area
Cuijue Slope Puzzle
To the southeast of the Cuijue Slope teleport waypoint, you will find a number of unlit torches. You only need to light the ones in the triangle, without lighting the others outside of it.
In the not-so grassy part of the area in the marked location, there are 3 training dummies. Destroy them to reveal a hidden chest.
Sea of Clouds, Liyue Puzzle
In the ruins above Liyue Harbor, you can find some unlit pillars with a spiral formation, which you can also see from the grass pattern. Light the pillars from the outside, starting from the end of the spiral, all the way inside at the center.
Northwest of that, you will find some small puzzle as well. There is an Anemo pillar in front of the cage doors. Light it, and go down by jumping from the small opening shown on the picture above. Then light the Anemo pillar below, and put down a Geo structure on the circle next to the pillar.
Lighting the Lantern
North east of the teleport waypoint, you can find an unlit lantern. You light this type of lantern by destroying a glowing rock that you can find nearby. Destroy the glowing rock that's on the other side of the tree and guide the light back to the lantern.
In the marked location shown above, east of the Statue of the Seven - Geo, there is a broken down house with a treasure chest on it. Outside of the house, near the bamboo area, you can find some piles of leaves that can be easily spotted using your vision. Get rid of them all using Anemo attacks, or Pyro attacks, and a hidden treasure will appear.
Guili Plains area
In the area marked on the map shown above, there are 3 boulders that are connected to each other from the ground, with 1 last area that has no boulder.
Use Geo main character and place a rock down with the elemental skill, to fill up the last spot to reveal a hidden chest.
West Guili Plains
There is an empty platform, with a boulder on the opposite side. Fill it with a Geo construct using Geo main character to reveal a hidden treasure.
To the west of the ruins above, there is a spot where the ice archer hilichurl is overseeing the other hilichurls planting multiple explosive barrels. You can detonate the bombs and reveal a treasure chest.
Dark Iron Sword
Go to the east teleport waypoint on Liyue Harbor, and talk to Chen the Sharp npc. Then select the "How're the fish selling?" option and continue the conversation, the npc will then gift you an exclusive weapon that you can't obtain anywhere else, the Dark Iron Sword.
It's 3 star sword with a unique effect that can also change its appearance when you get it to second ascension.
2 Luxurious Chests
Go up the stairs that's next to Mingxing Jewelry, that's marked on the map with a diamond icon, then go inside Northland Bank. Once inside, go up the stairs on the side and open the Luxurious chest to get 200,000 Mora.
Over in the distance that's to the right of Liyue Harbor, there is a big ship in the ocean that you can get to either by swimming, or gliding from the mountain that's close to it. There is a fairly short side quest that you can do once you're inside the boat.
Teleport to the teleport waypoint in Mt. Tianheng, then drop down to the right, entering Liyue Harbor. There is a precious chest, in the rocky area that's above the rooftops of the buildings in Liyue Harbor.
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