I have made lots of simple curtains, etc. Here's my idea: You have to choose between a topper that is quilted and just hangs straight across the window, with no gathers (I'm thinking not too much piecing) or do a pieced short curtain that is about 1 and 1/2 times the width of the window, which has a channel in it for a rod and hangs in light gathers.
If you want more of a sewing theme, there are lots of fabrics that have sewing notions, buttons, sewing machines, etc., on them. But the topic of the fabric might be lost, depending on how high the windows are at the top.
If this isn't understandable, let me know, I can try again!