Depending on what the weather is doing when you read this, some end-of-the-growing-season tasks can be done now, but others really need to be done later, once it is colder.
Generally speaking, any plants that will need to make it through the winter (grass, trees, and perennials) should continue to be watered until a freeze occurs. Fall is also a good time of year to clean up any plant debris and remove it from the area, especially if you have had disease or insect pest issues during the growing season. Other tasks are more area- or plant-specific: lawns, trees, flower beds, and veggie or fruit plantings all have their own unique needs.
For your fall lawn care, now is a good time to put herbicide on broadleaf weeds; do this before the weeds go dormant. Like all other perennial plants, they are sending energy into their roots to make it through winter, so they are more likely to also take in the herbicide. Use your lawn mower to chop up fallen leaves in your yard. Leaves add extra nutrients for your grass and add organic matter for your soil. As long as the leaves are not completely matted down or covering the entire lawn, they can be left on the lawn.
Some tasks you can do to help your trees: For winter protection against critters you may consider a variety of small “fences” — for mice/voles, hardware cloth that has openings of less than 1/4”, sunk a couple inches into the ground; for bunnies, fencing should reach 2 feet above the usual snow level; and for deer, protection around the trunk as high as you can go will keep them from rubbing the trunk. FOR WINTER ONLY (think, Thanksgiving until the spring equinox or Easter) adding some extra deep mulch will help protect tree roots from freezing in case there is not enough snow cover to do that. If you have young, thin-barked trees such as apples or maples, you may want to wrap them in white tubing or wrap to protect them from sunscald. Remember to take off any such protection in the spring.
In your perennial flower beds, depending on your plants and your gardening goals, some perennials can be cut down and the material removed, and some may be left in place to catch snow and leaves as a natural mulch, which can protect the plants’ roots from freezing. Some pollinators overwinter in stems or in the ground and they may also be protected by this technique. If you wish to save seeds to help propagate any perennials (especially native plants), you can collect them and either save them or place them in another spot where that plant is desired. Some seeds can be left for birds to enjoy. Leave plant material until the plants freeze or go dormant. As stated in the weed paragraph, perennial plants send energy down into their roots in the fall to make it through the winter.
In your vegetable or fruit garden areas, pull up or cut down any annual plants. Extra (shredded) tree leaves you didn’t leave on your lawn can be spread on the bare soil to keep down erosion and to add organic matter to your soil. Leave any perennial fruits/veggies (asparagus, rhubarb, strawberries…) until a freeze, then mulch for protection.