Nothing that is required or even mentioned in achievements (contains spoilers for all acts)
Some of these are obvious but ill list them all
-The skull can be looted for teeth
-The candle can be extinguished to grab a smoke from the bulb
-The clock at 11:00 gives a ring
-Placing cards according to the painting gives rewards
-The safe code is 273
-A few other secrets are linked to progression so ill leave them to be found however choose the arcane eye
Great Kraken card
By clicking on the tentacle found in two locations you’ll be given the card (any order)
The dock found under The shack at the top right of the map
The bath on the top floor of the wizard tower
This can be seen more easily with the monocle from the wizard tower, but if you visit the pelt location at the far left of the shack (top right) theres a secret path up from the area where you can visit the woodcarver for some lore
The totem (or whatever you wanna call it) thats found to the right of the Goo wizards second locations has an interesting secret.
The first icon is found by setting the clock to 11:00
The second is from spinning the drone found past the first bridge (click on the right or left side of it) Then clicking on the camera beneath it to light the wall behind it
I guessed the 3rd sigil after finding the first two but i believe its found by creating the creature found on the printing screen to the left of P03
The actual sigils are
insta kill (Skull)
double hit (twin arrows)
Mushroom boys boss
The mushroom hut on the centre right of the map can be entered (sounds obvious but somehow i missed this on initial playthrough) Inside you’ll find the mushroom boys asking for duplicates, it’ll just cycle between the same 4 with one from each faction. Each of which can be bought from the Trader by repeatedly buying the bottom left slot.
After a few merges youll get a key for later.
Heading to a similar area on the new map (go up one from the main shop and continue right till you cant) at this point there is a hidden arrow on the bottom right you can find by placing the mouse over it. This will take you to outside the hut where using the key can take you to a secret boss.
In the second phase they merge your creatures so be careful whats on the board however you can get some powerful cards if you can understand the mechanics of phase 1