Ok, I'm not the first to figure these out. Infact I have no clue who originally did figure them out.
A multiplayer easter egg
The Sign in the ocean on Zanzibar changes with the date.
Soffish (12/25 )
Baby Shark (1/1)
Ling-Ling's Head (7/7)
No swimming (All other days)
Carnage (10/31)
A Single Player Skull.
Difficulty: any Level:outskirts Skull Name: Blind
Effects: Removes wepon/ HUD
How to get it: After you reorient yourself (level should start off blurry), walk into the hallwayand turn around. You should see a flashing light. Jump on it then turn right and crouch jump to the roof. Then turn right agian and you will see some ammo and a Skull.