Cut back dahlias close to the ground and cover the bed with a thick mulch. Without extra protection, dahlia tubers will freeze to death. Or, dig the roots, let them dry indoors for a few days, and then store them in boxes, in a cool place indoors.
It's prime planting season for pansies, which grow best when set out early enough in the fall so that they are well rooted by winter. Small-flowered mini-pansies bloom better in cold weather, but use large-flowered types where you want a strong splash of spring color.
Spinach will keep growing for a few more weeks -- and stand strong through winter -- if you drench it with "fast food" such as fish emulsion or another soluble fertilizer. Even if your spinach is growing in good soil, cool soil temperatures may suppress the availability of natural nitrogen.
Water your lettuce often to help keep the leaves crisp and sweet. Harvest frequently, and prepare for the cold weather ahead by planning a protective box or tunnel for your lettuce, which can be installed quickly when the first hard freeze is predicted. In a pinch, a cardboard box will do.
Sow seed of bachelor buttons, corn poppies, larkspur, and ox-eye daisies. You can plant seed of black-eyed Susan in wildflower areas, too. Seedlings will sprout sporadically through winter, resulting in plants that bloom at different times the following year.