I like the suggestions of curtains/drapes that are permanently fixed at the peak and pulled to the sides to open. It’s a pretty window, and I think putting a permanent cover or valance on the top triangle would look weird. To me, curtains going to the peak look elegant. A triangle of fabric at the top looks cheap.
This isn’t a fair comparison because the only image I can find of a triangle valance has circus-color fabric, but it gives you a rough idea.
From the look of it, you don’t need it for privacy, it’s an upper floor and nobody but birds could peek in.
You might want to do sometime about the triangular part for room-darkening for sleeping, but I’d be tempted to wait to find out if that’s an issue with that window and the direction it’s facing. If it turns out you do want room darkening, there are hundreds of window films on Amazon in every possible color and plain or decorative effect. Simply cut to fit, it’s self adhering and you can easily remove it whenever you want. Probably less than $20. Then for the rest of the window below it, you can purchase a ready made curtain or drape in either a 54” or 63” length and install that in seconds with a tension curtain rod of the appropriate width. Amazon, Target, Lowes, etc for the rod. You should then be able to buy a ready made curtain or drape in that same matching fabric but in a longer length 72”, 84” or 90” for that other lovely window.
Well, these responses have been terrific! I prefer a blind to a curtain, particularly as it’s a pretty small window, and the blackout window film sounds super simple. Off to investigate!