The Reverie (Dreamscape) is the - currently - largest area in Penacony with the most number of treasure chests and puzzles. You can look at the official interactive map here to see the full layout and position of the chest (of course, this 3D map is also available in-game). In total, there are 48 treasure chests in this area, including puzzles and Lordly Trashcans. But I will not include the 12 treasure chests that you will get from the Adventure Mission 'Slaughterhouse.'
While doing the guides, the official interactive map didn't fully update so there are multiple pins that are not marked yet. So I edited the images and added red circles (#10, #11, #32, #33, and #48).
All Treasure Chests in Penacony (The Reverie - Dreamscape)
1. At the end of the corridor.
2. Inside the lounge, beside the stairs.
3. Complete the Dreamticker puzzle to unlock the treasure chest.
4. Floating on top of the carpet (and they look similar too) in front of a long table.
5. Defeat the Formidable Foe to unlock the treasure chest.
6. Behind the moving tiles, beside the mechanism to fix said tiles.
7. In front of Cavern of Corrosion.
8. Defeat the Formidable Foe to unlock the treasure chest.
9. Besides the elevator that will bring you to the VIP floor (same floor as Lady Vermillion).
10. Take the elevator to the VIP floor and you will see a Warp Trotter.
11. In front of the Warp Trotter you just defeated. Interact with the trash can and choose the correct option (Option 3 > Option 1). If you choose the wrong one, you will fight the trash can, but will still get the reward if you defeat it.
12. At the end of the corridor.
13. Enter the Arcane Artwork (painting icon on the mini-map) to reach this area.
14. Change into Hanu using Hanu's Adventure and use the rocket to teleport and reach the second floor.
15. Use the rocket to teleport and reach the bookshelf.
16. Beside the stairs, near a mechanism that unlocks a door.
17. Surrounded by enemies, near the Bubble Charge.
18. Defeat the Formidable Foe to unlock the treasure chest.
19. Near a locked red door (unless you had unlocked it), below the stairs.
20. Right after you cross a bubble bridge.
21. Behind the sofa in the lounge.
22. Complete the Dreamticker puzzle to unlock the treasure chest.
23. Behind the elevator near the Calyx.
24. Complete the Dreamticker puzzle to unlock the treasure chest.
25. On the second floor of the lounge.
26. On the opposite side of #26.
27. Complete the Dream Jigsaw to unlock the treasure chest.
28. On top of a really huge tile, near the bubble tower.
29. Near the many bookshelves.
30. At the corner of the wall (near a balloon with Origami Bird inside it)
31. At the corner, near the Stagnant Shadow.
32. Switch to Hanu using Hanu's Adventure, and use the rocket to reach the top of the bookshelf.
33. Still in Hanu's form, go to the opening of a bookshelf and you will see this treasure chest.
34. Inside the room.
35. Defeat the Warp Trotter.
36. After going down from the elevator, you can see this is in front of the lift.
37. Near the teleport waypoint.
38. Interact with the trash can. This time, there is no right or wrong answer and you need to defeat it.
39. Defeat the Formidable Foe challenge to unlock the treasure chest.
40. Complete the Dreamticker puzzle to unlock the treasure chest.
41. Behind the partitioner.
42. Complete the Dream Jigsaw puzzle to unlock the treasure chest.
43. Defeat the Formidable Foe challenge to unlock the treasure chest.
44. Under the stairs near the previous Formidable Foe enemy.
45. Behind the pillar in the main lounge (with a huge crash in the middle)
46. Under the huge balcony.
47. You need to cross lots of bubble bridges to reach this location.
48. For the last treasure chest, talk to the Dream Ticker Supervisor (you can see the icon on the map) and give them 15 Eerie Building Blocks. You can get those items after solving all the Dreamtickers puzzles.
For the last chest, make sure you read the message from '???' first or you won't be able to return those Building Blocks to the Dream Ticker Supervisor.