I think painting the walls does a great part, really shapes the room. But if thats not an option, you could get fabrics and hang them. In Europe/Germany there's a great shop, kamandala.de. And get flags at your countries antifa stores.

Another important part is indirect lighting. Really one light on top is absolute shit if you want it cozy, so some RGB Led bulbsand adjustable standing lights are IMHO great. Combining leds on wires with fabric/flags is great to let them shine at night. Works great on plants, plush, decoration, literally anything.

Personally I love photos, so I made a photowall with photos I took of cute plats, cats, friends and whatever felt neat to print+hang.

To be fair my vison of rooms is from videogames, specifically life is strange, buuuut whatever.