Jon,

Late August?  Best to worry about the heat and forget which film to carry.
Be sure to TAKE PLENTY OF WATER and buffered salt tablets.  On my June trip
to the bottom, it was 132 degrees along the river.  August is usually hotter
than June.  If you haven't experienced that type of heat, you are in for a
real surprise.

Rich

If you're doing it in one day then it's not smart to carry much gear, maybe a
body with one zoom and no tripod.  You really need to keep the weight down.
I've hiked rim to rim on the Kaibab maybe 18 times, usually overnighting on the
north rim and hiking back the second day.  21 miles each way if you stick to
the Kaibab, 4,800 ft down and 5,800 ft up the first day, then reverse the
second day.  23 miles if you come back up the Bright Angel trail.  Unless you
are a marathon runner who is fully acclimated to 105-110 F heat you can't carry
much gear or you'll wear out from heat exhaustion.  If you get heat stroke you
might die, something that happens 2-3 times most years.  August is not the
smartest time to do this hike :)  Mid-October, just before the north rim closes
for the season, is a lot better.

Basically start out on the south Kaibab as soon as you can see your feet, and
as you see interesting stuff along the trail, shoot it.  The light varys widely
so there are no set spots, but look for the mule train carrying food, etc to
Phantom Ranch heading down.  This train starts from the top around 6 AM and
shoot it if there's enough light with the canyon opening up to the east (you
should be 2 miles or so down when it passes you), and the two mule trains
coming up from Phantom Ranch with tourists, which you should see within a mile
of the river, if you started early enough.

Phantom Ranch is interesting and worth a quick stop.

The only detour I'd recommend is to Ribbon Falls, which is a beautiful
waterfall in two parts, first a 40 ft drop into a bath-tub sized basin, then a
60 ft drop down a heavily mossed up face.  In August the moss is always very
thick and sparkles emerald green as the water from the "tub" overflows down the
face.  You can climb up behind the "tub", or if you're really ambitious you can
reach the basin above the higher falls too.  This is about 1 mile before the
Cottonwood Campground, or about 13 miles from the south rim.  Great place to
eat an early lunch and cool off in the alcove, maybe wade in the pool, before
the death march up the North Kaibab gets steep a couple miles further on.  It's
only a couple hundred yards off the main trail, cross at the bridge and follow
the signs to the alcove.

A mile or two past Cottonwood Campground is the house of artist Bruce Aiken, a
famous painter whose day job is taking care of the pump that supplies water for
the south rim.  His kids used to set out a lemonade stand, which was a real
treat on hot days.  Past Bruce's house it's about 5 miles of steep uphill
hiking, pure hell if it's really hot.  Shoot it if you see it, but most people
are just slogging at this point, one foot in front of the other like a dumb
beast of burden, too tired to get the camera out.

If you are doing a multi-day trip with layover days at Cottonwood and Bright
Angel and Indian Garden Campgrounds, then a lot more options open up (grin).
Let me know if that's the case, there are many wonderful things to shoot from
each of these campgrounds on a layover.  But for one day in August you'll not
want to carry any more weight than necessary.  Make sure your canteens are full
of water when you leave Phantom Ranch, Cottonwood Camp and Aiken's house.
Usually there's water near a tunnel about 2.5 miles from the top, but not
always.  It's basically a three part hike ... 7 miles of steep downhill to the
river and Phantom Ranch (4,800 ft down), 8 miles of level hiking up Bright
Angel creek in high heat (about 1,200 ft very gradual gain), 6 miles of very
steep uphill hiking from the creek to the north rim (4,600 ft of up at the end
of the day).  Be careful.

Bill