You can use treasure compass to help find treasures that you have not claimed in the world. You get the blueprint to craft this by reaching level 6 in your Mondstadt's city reputation. If there are any nearby treasure, the compass will show you which direction to go, for 1 or 2 seconds, then enter a 30 second cooldown. If there are no treasures, then it's about 5 seconds cooldown.
After you get the blueprint, go to your inventory and "use" the blueprint. Then craft the item by going to the blacksmith.
Head west from the Stormbearer Mountains to the marked area shown above to find 3 unlit torches. Light them up with any of your Pyro characters like Amber or Bennett to reveal a hidden chest.
Not far south from the tree, there are pillars with elemental symbols on them. Use characters that can apply the needed elements on the pillars and a luxurious chest will appear.
To the west of the tree, there are some enemy camps and some more treasures.
Near the luxurious chest, head east to find an anemoculus near the pillar, and a treasure chest that can be spotted not far from it.
Luxurious Chest in Mondstadt
In front of the Cathedral, in the colosseum-like area, there is a small shining star near the stairs. Climb up to interact with it and some hoops will show up, guiding you to the other side of the railing. At the end of it, a hidden treasure will appear.
There is a second waypoint in Mondstadt, at the top of the building close to Knights of Favonius Headquarters. Once you reach the waypoint, you can see a luxurious chest next to it that you can reach by jumping and using the glider.
Free 4 Star Artifact in Mondstadt
Inside the Cathedral, talk to the nun Victoria for a free 4 star artifact Bloodstained Chevalier's Goblet.
Climb the statue that's in front of the cathedral and go to the edge of the hand. There will be an option to sit, where you will then get an achievement.
Chests around the Knights of Favonius Headquarters in Mondstadt
Sunsettia on a pillar
From the Midsummer Courtyard domain, head west to find a pillar like the one above. Pick up the fruit at the top of the pillar to reveal a hidden chest.
South of Midsummer Courtyard domain, there is an area with 3 Dandelions. When you approach it, an Anemo slime will show up. Defeat the Anemo slime and a hidden chest will appear.
In the area marked above, north of the colosseum in Wolvendom, there are 3 plant mines. Walk over the plants and they will detonate after a few seconds. After detonating all 3, a Cryo Whopperflower will appear. Defeat it, and a hidden chest will be there.
There are also 3 plant mines in Brightcrown Canyon. From the teleport waypoint, go south a little and you can find all 3 of the buried plant mines there. A Cryo Whopperflower will also appear after you detonate all 3, and a hidden chest will appear after you defeat it.
Thousand Winds Temple
On the edge of the colosseum in the Thousand Winds Temple, there are a couple of non-exploding barrels. Shoot/destroy all 3 and a hidden chest will appear.
Go to the southwest teleport waypoint in Stormterror's Lair, and go a little further southwest from it. Facing the direction shown in the picture above, drop down one level lower.
There will be some items that you can pick up in the area. Pick all of them up, and a hidden chest will appear.