Guide to make some braincells light up

BE WARNED, THIS GUIDE CONTAINS SPOILERS!

1. Monika being hinted at

There's a subtle detail on the game' official artwork, Monika has black socks instead of white, like the other 3.

Monika is the only character whomst' name doesn't end with an I (Unlike Yuri, Natsuki and Sayori)

2. Deleting any .chr files before the game starts

This only works if you haven't started the game yet upon a clean install.
Deleting any of the .chr (Character) files before the game starts, will result into a creepy image of Sayori hanging, actually, it's a gif.
If you wait 10 minutes, there will be text saying "Everybody can be happy now."

3. “hxppythxughts.png”

Appears after (SPOILER WARNING) the end of Act 1, in the game directory.
It's basically a sketch of Sayori hanging, highly distorted though.
There's a chance of a non-distorted variant of the photo to show up whenever you play the game, it's fairly common though.

4. Secret Monika Sticker

Deeper into the game, whilst writing a poem, there's a small chance for a secret Monika sticker to appear, next to the others.