Having just gotten off the Grand last week (while it's still fresh), I would never go again without the following:
1) Good pair of gloves - I used a pair of more expensive gardening gloves and they worked very well. Use them whenever you're rigging, opening dry bags, loading/unloading the boat, handling straps, etc.
2) Bag Balm - the incredibly dry climate will wreak havoc on both your feet and hands. Bring a different pair of gloves for night application, and, be sure to bring multiple pairs of cotton socks and slip-on shoes (Crocs woke well) for foot application and walking around camp.
3) A light colored sheet that you can soak with water in the daytime to keep your legs and feet out of the sun when you can't take any more. It is going to be sweltering! I also like the mister idea, that's got to feel pretty good!
4) Super Glue - worked ten times better on cracks/splits than NuSkin, which is more like applying a coat of Nail polish, and it peels right off!
5) Good splash gear - you will get wet, and the water is cold, so be sure you have a good top and bottom when you need them!
6) Wide brim hat (or two) with drawcord to stay on in the wind.
7) Light colored long-sleeved shirts and light pairs of pants to protect your skin from the sun. Sunscreen is definitely still needed, but soon, you will be one with the dirt/sand, and, applications become more exfoliating than as affective as they were at the start of the trip.
And, lastly, pack everything you think you need and then cut it in half. Then, cut it in half again (or, bring another dry bag for dirty laundry)! So far, since we returned last Tuesday, I have done 11 loads of laundry, and I still have 2-3 more to do (It was my first time down and I brought WAY TOO MUCH stuff!).
Other items that were nice to have: Solar charger, musical instruments (while you're usually pretty exhausted by the time you get to camp, it's always fun to play some music), sun shower, parawing or EZ-up for Camp Shade, solar landscaping lights on stakes which we charged daily and placed by the sand stakes so folks wouldn't trip at night, and battery-pwered light strings (like the ones they make for bicycles) - they're fun in the kitchen.
I know there's many more, but these are the things I would definitely be sure to take with me on my next Grand trip, anyway