Here is all secret content that I know of.
Guide to Easter Eggs
The game seems to come with a fully playable version of Pac-man, which can be accessed very easily.
Open up the dev console by clicking ` (the key above tab)
Type “map pacman_b8p” without quotation marks and hit enter.
After loading, your screen should look like this.
This is a bit of a stranger map than the Pac-man one.
If you type “map nonecludian” into console, then you should get a screen looking like this. There’s a bunch off odd areas made out of Portal resources.