Right before entering the Lostleaf Lake (still in the Cavern of Dreams area)



If you look upwards you'll notice a hole in the ceiling



Flying up there you'll find the Red Rubber Duck



I'm not sure, but i believe taking it out of there makes it also appear in the lake as shown below





You can find this in puzzle area with the Jester Boots



If you fly all the way up you'll find a hidden Green Sappleing, not sure if there is anything more to it




You can fly towards Raay in the game



You can find 2 of them in the Lostleaf Lake area, one of them being right in front of the lake



Here's him up close




Near the tree house, you can fly in the direction of the screenshot below to find it



You'll reach an area with giant trees and apples



To the left is where you'll find the tablet




Easy to miss, you can find it at the tree house




Inside the tree house, the ceiling has an opening



Flying up there you'll find the tablet




If you put out all the fires in his tomb area, you'll get a secret dialogue




On the pipes above the water, you'll see an opening



You can fly there to find giant statues (of Lady Opal's race) with a bunch of fish around, not sure if there is something here but i didn't find anything

It's in the area right after the steam pipe in the Cavern of Dreams, as shown below



Flying up towards the pipes, you'll find the tablet




In the puzzle area right before being shot up to the Airborne Armada, to the right of the entrance you'll find a pipe you can enter



Following this path you'll end in an area with a bunch of bars



Flying up you'll see a pipe entrance



At the end of the pipe, you'll see the painting




In the puzzle area right before being shot up to the Airborne Armada, you can fly up there instead of using the canon



Up there you'll find the giant ducks, not sure if there is anything else up there




In the area right before entering the Airborne Armada, after unlocking flight you can read the messages left near the ceiling which go about the military advantages of the Sappleings and Mushrooms






In the area right before entering the Airborne Armada, there is an opening in the ceiling



Outside you'll find 3 other silos similar to the one you are at



In these, you'll find hoops you can go through



Going through all of them in time, you'll unlock the Reapers in the area






Right in front of Kerrington, you'll be able to see a blue glow



Flying there you'll see a blue entrance with the Kerrington gliders to the side



Once inside, you can fly through the ceiling and find underwater hoops



If you go through all 3 in time, you'll unlock the FISHs in this area






Similar to the FISH area inside the blue house, this one is inside the red house (where you find the egg on fire).



Outside you'll find a set of 3 hoops on fire



Going through all 3 in time, you'll unlock the Dumplings in this area






You can hit his right eye to activate it



This also hints that his left eye is missing because he was attacked


In the green room with the Reaper, you can fly above the smoke ceiling



Up there you'll see an opening where the tablet is




In the Heart room, you can fly above the ceiling



Up there you'll see a platform with the tablet

This is in the area where you first find the fire birds



Flying up, towards the dragon statues you'll find the tablet




In the area right before entering the Prismic Palace, you'll find an opening in the ceiling



Going there you'll find the rubber duck






You can find this one near the observatory



Flying upwards you'll start seeing a few platforms you can go to, the tablet is in one of them




Across the tablet above, you can see the starfish.



If you fly to him, he'll greet you




On the platform with the Hidden stone tablet #2, you'll see an area with a frozen lake



Flying there you'll find a cave with the rubber duck inside




It's in the area with the Frozen King (and the scary box), the entrance is shown in the screenshot below



Just keep flying up to find the tablet




It's near the area where you disable the turrets



Go up the corridor, above this area (the one you went through to disable the turrets the first time)



Simply fly up to find another corridor, the tablet is at the end




If you put out all the fires in the area, you'll hear the sound effect you normally get by completing a puzzle



This unlocks a rubber duck below the bridge, but this only works once atm (not sure if that's intentional)

In the Dream area you find Sage's encyclopedia card, you'll see a few dragons in the distance



You can fly towards them and it's the same ones you traverse in Luna's boss fight



Also as a side-note there is a picture of Fynn in the sky




Right before entering the Gallery of Nightmares



You can fly up to the top of the castle to find the tablet




Near the entrance, above the trapdoor



You'll find it on top the platform above the trapdoor




Falling in the trapdoor and going in the dark area (the one illuminated by lightning strikes), you can find a giant door that is blocked.

If you heal Sniffles (the giant) by giving him medicine or using Fynn's water ability, he'll get excited and his jumping will break this door.



Behind this door, you'll find a house with Luna's diary inside (which explains her backstory and how Sage tried to raise her)



The diary hints that Luna created all the enemies you see, like the Keehee (firebat)




Behind the house with Luna's diary, you can fly above the bars and find a giant cut tree



I haven't really found anything in this area though...


You might have noticed that Kerrington is missing his left eye in the Airborne Armada area.

Also in that dark area, you can find a breakable floorboard in the underwater section to the left



To the corner, you'll find Kerrington's left eye (it opens up when you hit it)




In the same area above with Kerrington's eye, you'll find a weird blue starfish-looking creature. It follows you around but doesn't do much besides that



I speculate this is one of Luna's freaky creations (as hinted by her diary that she created creatures which Sage didn't find very good)


In the Gallery of Nightmares, you are able to throw medicine (or split water) to "heal" the painting in the "Green area" (swamp area) as seen below



This makes it easier to solve the puzzle since the arrows will be easier to see (although it's not necessary)


It's inside the painting of the "Green area" with the swamp shown above.
You'll find it on top of the moon, just keep flying up




Near the painting above, fly to the upper right corner to find it




Also near the painting you'll find Kerrington's gliders. If you bring them to Kerrington (Airborne Armada) he'll comment on you wearing it




It's inside the painting of the "Red area"



You'll find it on top of the snake, just keep flying up




In the "Red area" with the carts and lava, just fly upwards to find the message




Also found in the "Red area", the fish increases your underwater speed.
You can use it in the "blue painting" for an advantage




In the "blue painting" with the deep underwater section, you can find it right behind the cage




Also in the "blue painting", you can see these behind the windows



I'm not sure if the game that explains what the eyes are...they are similar to Kerrington's eyes though. So it's possible he was constructed using the eyes of this creature.


It's at the last part of the gallery, before the fight with Luna



Flying upwards you'll find an area in the walls with Luna's last message

In each of the areas, you can find tablets which explain the story behind the box

• Part 1: Lostleaf Lake
  
  
  
  
• Part 2: Airborne Armada
  
  
  
  
• Part 3: Prismic Palace
  
  
  
  
• Part 4: Prismic Palace
  
  

Conclusion: the box contained a powerful monster/magic, that on being opened froze time in the area it was in...this *technically* granted the King's wish of eternal life, but not the way he probably imagined.


Luna mentions how her father won't move and is very cold.



If we "visit" him, we can see he's looks just like Luna (also a bat).



Seems like the King ruined his city and himself by opening the Box, leaving Luna as perhaps the sole survivor. Sage then later rescued/adopted Luna.


In the Lostleaf Lake we find a drawing of him



Later finding him on Kerrington's ship




In Kerrington's ship we're shown that the Earth element creature grew too much and escaped his cage



Later in the Gallery of Nightmares we find Sniffles, which fits this description of growing too much

This is more speculation than anything, but given we have secret areas for the 3/4 elemental monsters (Fire=Dumpling, Water=FISH, Wind=Reaper, Earth=Giant), i wonder if there might be an area for the Giants too...?

OBS: after discovering the debug menu and going through the levels it seems this is not the case, but i'll leave this here just in case i'm wrong


In this area we can see a pattern where there are 4 dragons (Yellow, Blue, Red and Purple), each in one direction normally with a cave entrance beneath them



However the Purple Dragon breaks this pattern and doesn't have an entrance



I wonder if this is a hint that there is a secret unlock in this area

OBS: after discovering the debug menu and going through the levels it seems this is not the case, but i'll leave this here just in case i'm wrong (or if it it's a DLC foreshadow)

Credit for finding this to Pearbasket: https://psteamcommunity.yuanyoumao.com/app/2059660/discussions/0/3882724699534563722/

Simply access the Encyclopedia page via the menu and press H



It has a Level Select menu which include all levels (so you can replay for example the final fight if you wish) and also a few test levels

It has an ability menu, so you can for example turn on/off the flight if you wanna re-experience the levels with/without flight

Lastly it has a progression menu, where you can set the game to recognize actions that affect the main world. For example you can change if the game recognizes you helped Lady Opal.