First, let me convey my jealousy.

The road runs right along the south rim and the famous observation points
are all pretty easy to get to, and there are usually benches around.  Where
there isn't a bench you will probably find a low fence/railing that might
do.  You should be able to get to any of the major overlooks on the south
rim.  Don't bother to go out to the east end of the canyon (Desert View).
There can be interesting shots there (got one of my favorites there), but
parking is a significant walk from the edge.  I'm partially disabled - even
the handicapped parking isn't close.  The advice you got about Hopi Point
and Mather Points is right on -  both are great spots.  The South Rim Trail
runs all along the south edge and is an easy walk, but it's long and I'm not
sure how often you'd find places to sit.  I suggest you inquire about this
at the information plaza.

And I'm a bit jealous.

Early and late in the day offer the best light - by early I mean up to about
10 AM this time of year and from 3 PM on.  Like the man said, from 10-3
(midday), the light is pretty flat and tends to mute the features of the
canyon in photos.  If it snows, try to get out as early as you can - the
ground won't be very cold, and the snow will melt pretty fast, so your
window of opportunity for snow-covered shots will be limited.

Did I mention I'm jealous?

Have great trip, take twice as much film as you think you'll need (assuling
you're using film), and take it ALL in your carryon bag (NEVER put it in
checked luggage).

Bob in Las Vegas

Another suggestion is to stop in Tusayan (just outside the south entrance) and
see the Grand Canyon IMAX movie, especially for those who have limited
mobility.  I know it sounds silly watching a movie about a place you can see in
person 5 miles away, but it's a really good movie for the most part and you'll
get a good overview of the park.  The river running scenes re-enacting Powell's
trip are especially well done.