The Bathtub

Located in the bottom left corner near the entrance to Brawlbuster Fight, accessible using the slope, located near the entrance to Bomb Candy Mines.
Inside, you instantly break the bathtub, to which a microdemon says "You ruined my bath! That'll cost 1 quadraseptillion dollars, bub."
Upon re-entering East Wing, the bath will still be gone and he will have an empty textbox.

The Angered Gamer Nerd

Located in the bottom left corner of the entrance to the Pinball Mire, above the mud pit.
Access method is done in two steps:

• Use the slope from Entrance Hall that leads to West Wing.
  
• Jump over both gaps, leading to the room

Inside the room, there is a microdemon, sitting inside the recreation of The Nerd's room from Angry Video Game Nerd. He can say one of four things upon entering the room:

• "Aaaaaass!"
  
• "Wow, look out Micro Endy! You're going too fast!"
  
• "Antonblast? Yeah I'll blast alright, blast my ass all over this ♥♥♥♥♥♥♥ game!"
  
• "What were they THINKING?"

Brazil

Located to the right of entrance to Freako Dragon Fight, behind a cement mixer. You will see the room as soon as you will touch the wall.
Inside there are two ballbusters with guns, Christ the Redeemer statue with Satan's head, and a special yellow microdemon, themed after Brulo, that says "Welcome to Brulo's Brazil!".
An additional secret can be found here. If you manage to pull a precise jump, using the clutch boost, as well as using the statue as additional knockback (the statue resets when leaving the room, allowing you to take as many tries as you need), you can reach the top left corner of the room, where you can see a picture of Freako Dragon on his travel to Brazil.

The Rooftops

Located inside a wall to the left of Concrete Jungle entrance. It is one of the harder rooms to reach, as it requires to use the clutch boost, and then right near the wall use antomic bomb move to gain just enough height to reach the room.
Inside, there is:

• A Brulo microdemon, that either says "Did you know if you hold down after the Hammer Vault bounce, you can fall faster?", "Welcome to the Rooftops.", "Did you know you can send bombrocks n' stuff up higher by sliding?" or "Have you found the Aquarium?"
  
• A microdemon near a burger that either says "burmger" or "yum yum yum!"
  
• A microdemon that says "Have you met Greg and Matilda? I hate them."
  
• A microdemon in the corner that says "We all have stupid 'people names' to make fun of surface folk. I was gonna be Bridget, or Delilah, or like... Giulia. But I didn't fill out the form in time so they assigned one and now I'm... eugh... TINA."

Holiday Room

Located near the entrance to Freako Dragon Fight, accessible via using Antomic Bomb off the highest wooden ledge into the floor below. Inside there's a room with a green microdemon that says "Happy Holidays. Enjoy the greatest gift of all.", that gift being a red & green Peakman.

The Aquarium

Located to the right of the Ring A Ding Fight, inside the floor that can be destroyed with antomic bomb from a high distance.
Inside there is a room that leads to the aquarium itself. All of the fish are unique to this room and none of them respawn upon re-entering the room.

Antonball Deluxe

Located directly to the left of The Mad Mall entrance inside a wall, that can only be destroyed via a ball form. The only slope in the room that can allow you to destroy it is near the entrance to Smallbuster Fight, and the reliable way to reach to reach the room is to use the slope the instant the moving platforms rise up, then jump onto them and then slide down and jump into the wall.
Inside there is a corridor, where passing one of the pillars transforms you into Anton (or Annie) from Antonball Deluxe, even changing the controls accordingly. The pit at the end of the corridor leads you to the Antonball Deluxe room, where you can destroy the wall with shoulder bashes. You're not able to pick up the ball in the room, and it does not destroy the bricks. Nothing happens after you destroy the wall and the only way to leave is to jump back out of the room.
There is another secret in here - you can jump into the wall at the end of the corridor to find a hidden room with a microdemon from Antonball Deluxe that says "That's what they deserve for barely playing as the best character (me).

Skeletons in the Closet

Located in the bottom left corner near the entrance to Maulbuster Fight, accessible via the ball form, obtainable with the slope near the entrance to Smallbuster Fight, this time going up the platform and jumping on top of the floor. You can only access it after beating the game, as before that it will be blocked off via the black fog.
It leads to the hardest platforming segment in the whole game, separated with checkpoints. As its completion is out of scope for this guide, here's the video playthrough of the segment to see how you're supposed to beat it.
At the end you get the Tape, which currently does not actually do anything. If you did not buy every Tape from Brulo, it just acts as a free one, and if you did, nothing happens.