Fall is the perfect time to:
Collect seeds from your plants that have gone to seed. Sometimes this entails using a paper bag, and shaking the seed pods into it. Or placing a large sheet of paper under the plant, shaking the plant, and collecting the seeds from the paper.
Plant a Tree and shrubs and perennials. Fall is a great time to get plants established during mild weather and, hopefully, rainy days, before the often extreme heat of summer comes along and stresses them. It is not necessarily a great time to plant citrus or tropicals, especially as Fall turns to Winter and there's a significant danger of frost. But it is an absolutely great time to plant anything else. October does have it's warm, dry days, but starting in November things are usually cool and mild. I notice that plants planted in Fall may take a while to show significant growth, but by the following Spring the gardener is rewarded with a lush, happy addition to the garden. Fall is also a nice time to plant smaller plants, like 6" pots or 1 gallons, because by Spring you won't know the difference, and you will have saved a tidy sum.
Plant Winter Vegetables. Fall is the time when a lot of people figure their gardens will start to go fallow. You might have a couple tomato plants still putting out some good fruits, but things are looking raggedy, and it's probably time to fill up your green trash can and dream about Spring. Don't miss out on one of the best times to vegetable garden! I love growing during Fall and Winter. There are less pests, fewer weeds, and some of the watering is done for you! My absolute favorite thing to grow during this time is a salad mix. Arugula, lettuces, and other greens take so little care and can be grown from seed planted successively to give a long harvest. Everyone should try it, because it's easy and you only need a small space. Get a "Mesclun Mix" pack of seeds and carefully shake a small amount of seeds over a couple square feet of soil. Water by misting. They will be up and going within a week, and soon after that, you can start harvesting baby leaves. Heaven! Some other winter favorites: Swiss Chard, Carrots, Sugar Snap Peas.