The license plate on Hat Kid's scooter says 'H3T K1D 06'

In the bedroom, there is a secret hideout located under the mountain of pillows. If you use the ice hat above the light coming out of the pillow mountain, you'll land in the secret hideout. There is a diary in the hideout which changes every time you complete a level.

In the spaceship, there is a green door in the machine room (Which is initially blocked by crates that can be destroyed with the Brewing Hat ability), there is a gallery with paintings referencing real life paintings. There is also a time rift in there.

In Chapter 2: Battle of the Birds Act 2: Murder on the Owl Express, there is a screen in the room labeled 'Keep Out' showing the biology of Hat Kid, showing her skeleton and revealing the secret that she keeps snacks in her hat.

In the act 'Queen Vanessa's Manor' there is a hidden alternative solution where you can crawl from the cellar into the attic instantly.

The 5th act of Chapter 1 does not only have a time stop hat solution. There's even an alternate solution which is a bit difficult. Here's the walkthrough of this alternative solution:
-Run the other direction until you reach the beach.
-Jump into the cannon.
-Run to the finish line.

It is possible to go to the later parts of Dead Bird Studio during the finale of Chapter 2, except the stealth enemies will not load there either. However, the directors and the bill will still load.

In the room where you can access modded levels, there is a fan that gives you the Fan-Tastic achievement if you use a hookshot on it.

In the act 'Murder on the Owl Express' if Hat Kid's Uncle's Sister is chosen to be the murderer, it is revealed that DJ Grooves pretended to be Hat Kid's Uncle's Sister. He then tells the Conductor that he pretended to be Hat Kid's Uncle's Sister to help with the plot of the movie.

In the act 'Murder on the Owl Express' when you obtain the Crime Solving Outfit, you cannot leave the train's front room until you answer the phone. The same thing also occurs in the Spaceship after completing the very first part of the finale of Chapter 2 if you attempt to leave the Machine Room.

In the first act of Chapter 2, if you use the glasses machine 3 times, the machine will break and you'll be billed for Destruction of property (The same thing that occurs when you jump on the first three sandbags) with a fine of three thousand pons. I'm not joking. Here's an example of this happening:

If you complete the game, a Completionist's Hat can be found in the laboratory connected to the Basement (Which can be accessed if you use a Dweller's Mask in front of the purple object blocking the way). This room also contains the 4th Relic Stand and a time rift.

If you press 4 or Down on the D-Pad, Hat Kid will smooch anything in her way. If you press 1 or [What xBox Control must go here], Hat Kid will imitate anything in her way by making a face or blowing a raspberry.

In the finale of Chapter 2, the loading screen for the last part will vary on who originally won Award 42.

You get this loading screen if the Conductor wins:

Moon Penguins somehow possess unusual abilities for penguins such as being able to hover for a short period of time and snap despite having no fingers. This is shown on the wiki.

Science Owls were unused characters in place of Express Owls that only wore lab coats. Their main role was supposed to be in Train Rush as enemies, but they were replace with Express Owls. Their attack was throwing a clipboard at Hat Kid.

Surprisingly, Science Owls still make appearances.

Tim, CEO of Time is a scrapped character who appeared in the Pre-Release and Prototype versions of the game as a supporting character to help Hat Kid restore time.
More Info on Tim, CEO of Time[ahatintime.wikia.com]

Thor, one of the frozen victims in Queen Vanessa's Manor, was another cut character who set up all the robots and hookshot points in Mafia Town. His cap was also an early version of the Sprint Hat.

If Hat Kid stays in her bedroom for 20 minutes, a disturbing noise can be heard.
Watch this video:
https://www.youtube.com/watch?v=suOSDlkNpWc

The Hunt For The Roach King is a misc. thing you can do on the Spaceship, starting at the bedroom and ending at the Attic. You need to use the Camera Badge to find the notes.

One of the Mafia goons in the final act states that it's always 5 o'clock in Mafia Town if Hat Kid talks to him.

If you use the Dweller's Mask in the corridor leading to the bedroom, you will see drawings on the walls.

Behind DJ Grooves's glasses are his eyes. It is revealed if you use the camera badge to see through his glasses in the direction that is the opposite of the way he's facing.

Breaking the game is one thing that's possible in the game by going into other act areas. For example, going into the main part of the fourth act from Acts 5 to 7 in Chapter 1: Mafia Town.

In the Mafia HQ, there are 2 things you can figure out with the Camera Badge. One stall has red in the toilet.

Another stall is occupied by a Mafia Goon. There is also a 'S' in the same stall toilet.

Every boss has their own health meter, as shown below:
Mafia Boss: 10
Conductor/DJ Grooves: 29
Toilet of Doom: 12
The Snatcher: 5
Mustache Girl: 28

In the act Dead Bird Studio, if you don't move at all when getting your photo taken, DJ Grooves will say that Hat Kid isn't very photogenic.

Some events in levels are mentioned at other points in the game, for example:
-Mustache Girl saying that both her and Hat Kid will enter the Mafia HQ after act 1-2 (To Act 1-4)
-A Mafia member saying he remembers when Mafia Town was struck by lava (To Act 1-6)
-The Conductor/DJ Grooves heart to heart and some parts of the boss battle (To various acts in Chapter 2)
-Mustache Girl planning to take all the time pieces for herself (To the 5th chapter's prologue)
-The Snatcher telling Hat Kid how the old contractor's head was popped off before snatching Hat Kid's top hat (To Act 3-1)
-DJ Grooves and the Conductor telling Hat Kid to attend their movie screenings the next day (To Acts 2-2 and 2-3)
-The Conductor or DJ Grooves warning the other director that they know the power of the Time Piece (To Act 2-6 part 2 during the call after the award ceremony)
-The CAW Agents asking you for your uncle's sister's maiden name, your pet's name and which body part you are most ashamed of in act 2-2 (To other points in the same act)

If you look closely at the faces of Mustache Girl and the Mafia Boss, both of them have the same mustache shape.

If you beat the Death Wish 'Rift Collapse: Deep Sea' you'll unlock the Mirror Badge, which mirrors everything (Except the HUD). To the right is the icon for it.

In various acts, some NPCs figure out some stuff about Hat Kid, such as what she does.
-How did Dead Bird Studio's directors know that Hat Kid lived on a spaceship? (Act: Dead Bird Studio)
-Multiple characters were hanging onto Hat Kid's Spaceship. How'd they all know it's location? (Act: The Finale)
-How did Mustache Girl, Cooking Cat, the Mafia Boss (As a jar), the Wowl Band and the Snatcher (Snatcher only is there if you have the Seal the Deal DLC) get inside Hat Kid's spaceship? (Various acts)
-How did the winning director know the powers of the time pieces? (Act: Award Ceremony (Part 2))

If you've seen that weird dance with the smug face in some artworks and screenshots, I learned how to do it on an artwork where Hat Kid and Bow Kid are doing it in front of laundry detergent: By spamming the 1 and 4 keys on a keyboard or Up and Down on a controller.

This interaction has also become a pretty popular meme.

You may know that Eileen Montgomery voices the mustache girl. If you view the credits, you may notice that Eileen Montgomery also voiced Queen Vanessa. Similarly, the Bow Kid's voice actor is the same as Hat Kid's, Apphia Yu.

Did you ever try to find out which accent The Conductor uses? Some people have screenshots, artwork or videos using the word 'Scottish' or 'Scotsman.' Thus, the Conductor has a Scottish accent.

The Mirror Badge in the StD DLC has a reversed description which says "Dnuora gnihtyreve spilf!". If you look closely, the description can be decoded as 'Flips everything around!' backwards.

Some acts can contain instant death traps. Examples include:
-In the Train Rush, Award Ceremony and Rock the Boat* acts, if the timer reaches zero, INSTANT DEATH.
-Refusing to sign the contract in the act Contractual Obligations will cause the Snatcher to instantly kill you.
-You can attack the Empress in Nyakuza Metro**, but doing so will cause her to instantly kill you in one hit.
-In Queen Vanessa's Manor, if Queen Vanessa reaches you, she will instantly freeze you.

* Requires Seal the Deal
** Requires Nyakuza Metro

You have wanted to start the finale for Nyakuza Metro after collecting the 7 other time pieces, right? Well, here's the solution I figured out:
-Jump past the Empress's counter.
-Go through the door.
-Steal back all time pieces in the vault.
-When the door closes, walk to it, then the act will start!

If you have both Seal the Deal and Nyakuza Metro installed, you can start a fresh new save file as Bow Kid.