The quickest and easiest way to ring the seasonal changes with your window boxes is to fill them with individual plant pots. It makes it easier to switch out a plant that is not doing well or needs refreshing.
For fall I love ornament kale. The rich purple hues look fabulous with grasses or branches of berries to add height. Alternate them with miniature pumpkins or orange pansies. Use decorative moss or branches of leaves to hide the pots.