A simple and fairly effective "treatment" you can do on the windows that doesn't involve some sort of construction or permanent covering...is to hang extra thick drapes...I mean, the heavy shit that's almost quilt/tapestry-like.
If you have someone who can make drapes for you, there is soft/flexible acoustic "blanket" material that you could insert inside a custom sewn set of drapes.
You can find it on acoustic supply sites.
Of course...you can also cover the windows with typical broadband traps...but plan on that being a more permanent cover.
Now if you have major traffic noise and/or industrial noise...or just very noisy neighbors...you will need a whole lot more to kill that sound off, but I find that in typical neighborhood settings, it's not a real issue all the time. I mean, maybe when the neighbors are cutting the grass, you will have to wait...but generally, hearing a bit of outside noise with your ears will not be picked up by your mics...unless you have them gained up and facing the windows...plus, while your are singing/playing into the mics, your audio signals will be about 10 times louder than the outside noise...so that outside stuff will be masked out.
I have three 6' (almost 2 meter) wide windows in my studio, all double-pane...looking out into my yard and gardens, and I wouldn't cover them for nothing, as I love the natural light and the view.
I live in a real quiet wooded residential neighborhood, so I only have some noise if someone is cutting grass or using a chainsaw...but TBH, I've never had it bother my recording...it's just too distant and faint.