Of all the seasons in the year, I think autumn is my favorite time to fish. The temperatures are cooler, the foliage should be stunning, and many folks are hunting rather than fishing.
If you want to introduce someone to fly fishing, autumn is a great time. Some rivers are stocked by Columbus Day. Connecticut started stocking last week (updates here), and Mass. DFW usually starts late-September (updates here).
Also, if you yourself are new to fly fishing, there are many rivers that offer easy access. Our blog has written overviews on some rivers at which you easily can DIY without a guide.
But, my #1 autumn suggestion for new anglers is the Swift Hatchery Pipe area. Easy access, simple wading, and loaded with fish before catch-and-keep returns January 1. When brookies spawn, the eggs in the drift really get the rainbows going. My fishing notes indicate high-volume outings there in November.
Enjoy the autumn!