Coo is an old Minecraft world based on the old debug map, which is why there are so many floating islands.
The spawn point is just outside the only building in the map, which is where the adventure starts. Once inside the house, the player is told he is stuck inside and must find a way to escape without breaking any blocks and without dying(I used glass blocks not knowing that they could be destroyed in Adventure mode becuase they look nice). There is only one path out of the house, but be warned: the path is riddled with traps, false endings, forks, and little puzzles.
The server for Coo is currently not ready yet. Tommy wants to have some YouTubers (such as Robbaz) play on the server, so this may be the easiest way. Plus, people who want to play wouldn't have to download anything.
Download the source by cloning or grabbing the latest release.
Move the files (extract if you downloaded the zip or tar.gz) to %appdata%/.minecraft/saves (on Windows) or ~/.minecraft/saves (on Linux/Mac), open Minecraft, and select Coo.