A park. The Nuka World Among the Stars vault had one, I think, but I don't recall anything similar in any of the actual vaults. A place with benches, trees, and plants. A place for the people to just hang out with the color green. If it needs to provide a game function, perhaps a happiness bonus.

To segue into another game, much of my game play in Minecraft is underground and even there I'll make a green area much as described above. It just exists as a place for non-existent 'vault dwellers' to chill out. Of course, along one of the far walls are usually villagers who have been selected from the zombie rehabilitation facility and then imprisoned in 3x3 meter cells to serve as traders...