I'd like a window treatment idea that isn't curtains.

I am curantly obsessed with the Macrame craze that's going on

https://abeautifulmess.com/2014/02/make-your-own-macrame-curtain.html

https://www.pinterest.com/explore/beaded-curtains/?lp=true

If your walls are fairly smooth, you can use Command brand hooks to hang light weight fabric (i.e. sheers, large scarves) or even to hang a cafe rod (I assume you don't want to put holes in the walls?!) To allow light weight curtains or valance. Enjoy your new apartment!

What about window film? It comes in an array of patterns and colors and is removable without causing damage. This is a good option for still allowing light in. Also, you could use a tension rod to hang curtains - no hardware required.

I used pleated shades from Home Depot. There are no strings. Pull on the bottom of the shade or lightly push up to open. Now my beautiful stained woodwork is showcased! And only a couple of screws at the top of the window trim.

Measure inside the window casing—measuring width at top, middle and bottom of window. Of the three numbers, write down the widest number and that is width. Do the same for length. i took the widest and the longest number to Home Depot. They helped me determine what length and width to buy, then they cut the width to the size I needed for each window.

The dept manager had us go back home and remeasure, even though we had just installed new windows, saying the actual murasurements may be off. She was right!

Good luck With your project!

You can take glass paint and make it like stained glass. It can be scraped off

You could use tension rods to hang lightweight curtains.

How about a tri-fold screen. You can make one yourself with some 1"x1" wood strips and hinges. You can put paper on it or fabric, and fold them to the side.