If you can only buy one thing for an empty room, buy a lamp. Not a couch, not a rug - a lamp. It sounds too simple to be true, and it is the most reliable advice we give.
Overhead light is the problem
The single bad ceiling light in most apartments makes any room look like a waiting area, no matter what is in it. It flattens everything and casts hard shadows.
Light at eye level fixes it instantly
A lamp at eye level, with a warm bulb instead of the blue-white one it ships with, softens the whole room. The same empty space suddenly feels calm instead of clinical - and it costs about thirty dollars.
Warm bulbs, every time
The single cheapest change that makes a room feel like a home instead of an office is a 2700K warm bulb. Swap out the harsh one the fixture came with and notice the difference that night.
Lighting is the whole idea behind the Lights On kit - it is the cheapest thing on the site and it changes how a place feels more than furniture does.