There are many flowers that are at their best in the fall. Timing is the problem. We can never guarantee exactly when a plant will be in it's best bloom, so if you are planning on growing them yourself, you will have to plant a variety and hope for the best. You also may want to involve friends or relatives who have gardens with perennials and shrubs that will work. Of course, there is always the rose. Most rose bushes produce a beautiful display in the cool weather of fall. Likewise, most hydrangea shrubs are at their fullest bloom at that time with colors that range from green to white to pink to mauve.
Annuals include bachelor's buttons, Calendula (pot marigold), snapdragons, and sunflowers. There are so many wonderful varieties of new sunflowers that you may want to consider using just sunflowers in their many shades of umber, gold, yellow, and russet. Oriental lilies are quite often in bloom in the fall too, but that timing is touchy.
Many perennials bloom in the fall: Rudebeckia, asters, chrysanthemums, Capanula, Chelone (Turtlehead), Joe Pye weed, Helenium (sneezeweed), goldenrod, and sedums such as Autumn Joy, Autumn Fire, and other upright varieties. If someone has baby's breath in her garden, it too will probably be in bloom at that time. To keep flowers coming, it is necessary to keep cutting off the older ones, forcing the plant to produce new flowers. You need to be prepared to go with whatever looks good at the time.
