Someone mentioned not having found anything to do with old CDs. Hearing that apple farmers will put reflective strips in their apple trees, to blow in the wind and scare off birds and wondering if some such would work in a cherry tree, I started a new project.
I create cords using kumihimo, from 3 to 6 feet long. One end gets flattened out and glued between two CDs (reflective sides out). At the other end I 3-strand braid the ribbons/cords into two cords. That end I tie to a branch in the tree, letting the CDs dangle. They do blow in the wind and, in the sun, cast reflections. I actually think they look quite nice, hanging in the tree. And the birds quite enjoy watching them as they eat the cherries.
Still, one way to re-use old CDs.