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
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