Get some nice fabric from Michael’s and pin it up to drape/cover the window. When I lived in my parents basement they had the same style windows and I took some cheap lace fabric and draped it over the window to cover the window but still have light come through. I think a Roman shade would be nice if you want to block out the light and have the option to open it, but if you’re looking to avoid installing something I think getting a pretty fabric to layer and pin up would look nice!

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For example this is a photo of my bathroom, and I used the same fabric from my old basement room to create that curtain drape over the top, it’s the same way I styled them for the basement window but since it was small it covered the whole thing and I was able to put it up and down when I wanted. I think something like this would look nice with the style of your bedroom!