Although I haven't been to Yosemite in fall, I've been in spring and summer and have enjoyed it immensely in both seasons. Summer, with the crowds and flat, cloudless skies is arguably the worse season for photography, and yet I just got back from a trip earlier in July and found many wonderful photographic opportunities. Looking over my photos, my wife commented "I guess it's basically impossible to take a bad photograph in Yosemite." (not that she expected me to take bad photographs, there just were remarkably few we wanted to trash.)
One of my favorite hikes is to take the Panorama trail. It's easiest going down hill. You can take the bus from the Valley to the top of Glacier Point, and then walk 8.5 miles mostly downhill along a ridge with dramatic views of the Valley and Half Dome, then past Illouette, Nevada, and Vernal Falls. I've done a fair bit of backpacking in the Sierras, and I think it one of the most spectacular hikes anywhere. (You can also hike up the Panorama trail from Happy Isles to Glacier Point, then snake down the steep Four Mile trail back to the Valley floor).
Before my visit this summer, I found several books at Amazon about photographing Yosemite. Others on this site have said "f8 and be there" and I think that *being there* is key. If you enjoy the challenge of trying to make "postcard quality" images, arming yourself before hand with the knowledge of where to be and when can be invaluable. My son and I enjoyed planning for each day where we wanted to be for sunrise and sunset.
I also enjoyed meeting other photographers there. I spent a while talking with one whose hobby was to study the work of Ansel Adams, then return to the places from which Adams had made his images and then make his own interpretations of the same scenes. He had spent years following in Adams footsteps and had many insights that I was glad he shared.
My favorite place to stay is Yosemite Lodge. The Awahnee is beautiful but pricey; Camp Curry is a bit rustic -- I like the comfort of the Lodge, but perhaps I'm just becomming a softey.
Have a great trip. I am sure you'll enjoy yourself.