We started with 2 20" by 28" paintings, which wasn't large enough. I was originally thinking macrame (knotted yarns) for mine. Then we saw a 5x7' rug that fits perfectly. It's kind of an Indian Rug design. If I were choosing today, I would probably choose the design that looks like hanging square in a squares.
For your daughter, it depends on what her tastes are. Based on our experience, try hanging just a piece of fabric there to be sure that 28" square would be large enough. Then probably a single block done in colors that would complement her decor. I like all of the suggestions so far -- bargello, carpenter's wheel, Labyrinth, Crystal Star,