How can I “spruce up” a wall in my bedroom without painting it?

Hang a co odernating piece of fabric on the wall

What about hanging a large tapestry? It's a great way to warm up a room

Maybe a grouping of pretty pictures. Or add a shelf for pretty items.

Removable wall paper?

Perhaps removable wallpaper...or wall stickers? I've seen both on Etsy.

Lattice or bead board would be cute. You would need supports to hang. You may make a few holes in the walls.

What I'd do is change out the curtains (use white sheets and paint, use curtain clips to hang on rod), buy white pillow cases and paint fun colors and use as pillow covers. I'd buy throw rugs at the dollar store and paint too. A whole new look and no worries about how to secure anything to the wall.

Here are some ideas

https://www.pinterest.com/search/pins/?rs=ac&len=2&q=wall%20decor&eq=wall&etslf=NaN&term_meta[]=wall%7Cautocomplete%7Cundefined&term_meta[]=decor%7Cautocomplete%7Cundefined

There is some paneling with a wallpaper on it. White with little blue flowers! If that was something you liked? You could attach it to the wall with a few small nails and maybe some two sided tape if necessary? You wouldn’t need to nail each sheet much. Just a litt at the top couple in the middle and a couple on the bottom. You also could put your own 1/4-1/2 plywood on the wall and just enough nails to keep it standing against the wall. You could then paint the boards or you could texture the boards to look like the wall then paint too! Then you wouldn’t be painting the apartment walls!

Peel and stick tile, also easy peel contact paper.