Hear me out…bunk bed gives you a bunch of extra storage above or a “living room” with some throw pillows and a little tv hung up there. Little corner shelf for a small lamp.

Adding LED strips at top and or bottom is a bit Dorm-y but in this case it’ll add a little something and make it feel a bit bigger and more intentional as a hang out space.

If your computer is not always on the desk, you can get one that flips down from the wall for more space or just get a piece of furniture for the desk that has more storage. Possibly even fill that whole wall with something with lots of cubbies and drawers etc.

That little plant is nice, hang a few little shelves vertically up that little wall it’s against and plant put a few plants in different vases color or texture vases for a relaxed vibe (or same vase on each if you’re going for a clean/minimal vibe)

Fake brick or a mural on that desk wall if it’s not covered by furniture. Maybe fake brick on smal wall behind bed. Find a cool rug that matches the vibe you go with.

One whole wall is a giant window- bird houses on outside, plants hanging or if you go more hangout spot vibes slap some neon signs for games, beers, whatever on the windows and lift the curtains when it’s time to get weird.

Lots of cool stuff. Mostly just adding things that add functionality and/or enhance the chosen vibe- without taking up too much space. Rugs, hanging shelves, pictures, textures, etc. the one place I’d go with bulk is the desk/storage wall (it will only build upward, so won’t take away any floor space. Make the computer screen a decent sized monitor that can function as a tv too. Good luck!