Do you have a wonderful old tablecloth or possibly a piece of fabric or lace that you love? I love to see these vintage linens up on the windows with the light peeking through. I found a few photos to share with you of simple lace used as treatments. You could easily buy some clip rings like I have pictured here.
Just fold your tablecloth or linen to the desired length (play with this) and clip the rings evenly spaced apart and instantly you have a gorgeous curtain and you haven't harmed the fabric at all! You can use table runners or dresser scarves also. At the antique store I just used a nail at each corner and placed an open part of each corner of the lace over the nails resulting in a pretty valance.
I love the way these are gathered at the bottom and are slightly raised off the sill. This is just several rows of those tiny white plastic rings sewn on the back about 6 inches apart up the fabric. Then you can use some string to tie each row together and they stack on top of each other! These rings can be easily removed if you decide to change your window treatment.
Another simple idea is to take a twin chenille bedspread and fold it in half and drape it over a rod (this covered a double window). About 1/4 of the way in on the right and left sides I used ribbon ties starting at the top then gathered up the fabric at the bottom some, then took the ribbon back up to the rod and tied it there. You do this on both sides and it turns out to be a beautiful butterfly effect (similar to the photo above) with a gorgeous drape in the center, and you've not harmed the fabric at all!