I too am hiking the canyon in August.
North Rim to South Rim mid August.
As far as underwear chaffing, I use:
http://www.rei.com/product/694431
They are pricey but are worth it.
I rode 3000 miles across I-10 last month on my motorcycle to try them.
When I got home I bought 2 more.
I also cut the liners out of my shorts.
The hike should be fun. It will be hot and 2 ltr of water should get you to the next water stop.
Some other items to bring for the hike is jerky and other snacks.
Take your time and don't try to be the Tough Guy and run it.
With you bringing a camera it will make you slow down a bit which is good.
August can be hot exceeding 110-115. Just remember to hydrate and salt tab or salted snacks. Not too much though.
BTW when are you dropped off to hike out?
Have fun.