A hidden quest can be found by talking to Idyia after completing the Secret Summer Paradise: Part 3.
➥ Teleport to Silver Bottle Courtyard west waypoint and head east towards a booth with Idyia in it. There won’t be a quest indicator on top of her so you need to interact with her to start the quest. You need to find 3 treasures using the cipher mentioned by Idyia.
➥ For the first location of the hidden treasure, follow the quest navigation until you reach two sets of tables and chairs. Sit on the chair nearest to the lamp at the left table in the highlighted area on your map, and the chest should pop up. If it doesn’t, try another chair at that table until it does. There is no Joyeux Voucher in this chest and just the treasure for the quest. Report back to Idyia after this.
➥ For the second location of the hidden treasure, follow the quest navigation until you reach a Choo-Choo Cart. Board the train (Paimon should mention how you haven’t activated it before) and once in the tunnel, your area will be blocked so use the Preprint puzzles on the wall to unlock the blocked area.
➥ Make sure to push the box to the right side before starting the Streaming Projector and complete the Preprint puzzle. After completing the puzzle, you will get 1 Joyeux Voucher.
➥ Now that you are on the other side of the vine, dig the ground at the shining spot to get the second treasure. Report again to Idyia after this.
➥ For the last hidden treasure, the puzzle is quite long. Follow the quest navigation and go up to 3 Eidolons surrounding a bubble and play hide-and-seek with them, their hiding area will be highlighted on the map.
➥ One will be surrounded by enemies you have to defeat, another one will turn into a Bloatty Floatty puzzle, and another one will be inside the tree trunk area and you’ll have to chase it around and then talk to it (Make sure you talk to all 3 Eidolons with the option ‘Please go back to the original spot.’
➥ After you get all 3 Eidolons, they will create Four-Leaf Sigils, follow the sigils inside the tree trunk up until you’re close to the large vine circly-thing(?) that is rotating in a circle like a ferris wheel.
➥ Wait for the platform that looks like a floating leaf with an umbrella to circle around and jump onto it.
➥ Ride it all the way to the top until you can fly to the closest Four-Leaf Sigil, then take the sigils to the floating island. The curio will be in a Luxurious Chest you have to dig up (there will be an option to dig at the center).
➥ Once you report to Idyia, you will get an option to bury your treasure. I choose the option to bury it ‘a the entrance of the Veluriyam Mirage ‘Someplace high up in the Silver Bottle Courtyard.’ It doesn’t matter what you choose because you will get the same item anyway.
➥ When you reach this location, there will be two shining spots. The first one is to bury the treasure, and another one you can dig out a ‘Mysterious Mora Pocketwatch.’ You can talk to Idyia again about this item but there will be no more continuation after that.