Underground Outpost 45
This away mission is about an abandoned nanotrasen-owned outpost built into a cave, which is also a gas pocket. Its primary purpose was leisure with its large amount of windows, large hydro bay and bar, and multiple dorm rooms. It also included a small research arm (because how else would you get back) and an elevator to the surface (which does not lead anywhere currently).
The base is fully functional and possible to take control of fairly easily, although different access requirements there mean nothing you bring from the station short of a cyborg or AI can easily open the doors to some of the more restricted areas. If nothing else, it offers a small atmospherics area that you can tinker with (and flood the outpost with deadly gasses), a mining operation that lets you bring minerals back to the station (or continue powering the outpost), and an R&D lab totally separate from the station's network (syncing either will not affect eachother, though tech disks can be brought over).
Here is a SPOILER INTENSIVE in-game image of the map, with lighting making it look very moody. Note that the items appearing above certain lockers have since been fixed (carn rewrite had something to do with it).