–sequins (we used a pack with shades of blue and silver)
Push a sequin pin through the sequin into the foam ball and repeat this until the entire ball is covered in sequins. When done, add a ribbon on top using a pin and hang on the tree.
A few of my kids made mosaics with their sequins in patterns that created fish or faces or other shapes. You could also vary the look of them by using different sizes of foam balls or by using foam eggs.
*of course, since this craft uses actual pins, adult supervision should be used with younger children.