Lots of good info from eeek and Bruce, so I don't have that much to
add.
120 is the best route from the north and west. Do obey speed
restrictions and limit your speed on Priest Grade. If you do that
you should be just fine.
The weather has been pretty nice. You should plan to dress in
layers and be prepared for anything from 30-80 degrees.
There are shuttle buses running in the valley. There are buses
to Tuolumne and Glacier Point, but I don't think they are free.
They may not be year round.
You asked about things to see and waterfalls. Yosemite Falls is
dry. Vernal and Nevada falls run year round. As of 10/21 Bridalveil
Falls was still active.
If you can get a shuttle to Glacier Point, and you are a good hiker,
Panorama Trail is an option. It is a long haul, but worth it! Bring
a camera and lots of film/memory cards! Also bring a rain shell or
waterproof jacket. It takes you from Glacier Point down past
Nevada and Vernal Falls. Awesome views. Bring lunch and lots
of water. Early start mandatory.
If you can't get that bus, you'll want to hike those falls from the valley.
The free shuttle stop is Happy Isles.
I suggest buying a good book of hikes. There are many of them.
If you don't have one when you arrive you can purchase one at
the Visitor Center store in the valley. I like Jeff Schaffer's books,
but reading them you may be disappointed that you can't hike
that many trails before you leave!
Other 'don't misses': view from Wawona Tunnel (or Inspiration Point),
views of El Capitan, and a visit to the Ahwahnee Hotel. Best time to
view the valley is just before sunset.