Here's a list of stuff you need for a summer trip (and YES, you do need the cool weather gear). The Canyon is the desert and the air temp can be cold at night/when raining, plus, the river runs 44 degrees, so don't forget the Polypro! The rule of thumb is bring less than more. You won't need much but the clothes on your back. Plus, anything you bring will be trashed by the end of the trip so shop accordingly.
Cool/wicking shirt (3) plus at least one long sleeve white shirt for sun protection
Sun Hats (2, wide brim to protect head, face, and neck, preferably crushable, with some sort of tie strap for wind)
Jog bra type swim top (2)
Shorts (quick dry, not cotton) 2 for river, 1 pair for camp
Bandanas (wet them and tie around neck, blowing sand protection)
Underwear (2-3 pair)
Socks (3 pair)
Light sun pants (loose, quick-dry)
Sarong: This is a lightweight cotton wraparound for wearing in camp, on the boat, or hiking. Very comfy, versatile, just get 45-50" of fabric in your favorite pattern, wrap around your waist, enjoy.)
Polypro/capilene top and bottom for river
Splash/rain Jacket (wind/rain/COLD river water dowsings)
Fleece jacket for camp
Post-trip change of clothes to leave in car
Needle and thread/safety pins
Broken in River Sandals, 2 pair (MUST have good gripping soles for climbing and stay on feet if you have a swim)
Sneakers/socks for camp, dry hikes (OPTIONAL)
Stuff for Phantom Ranch: Stamps, Envelopes, Address (for postcards) , Money (for Lemonade and Ice Cream), calling card # (for payphone, no cell service at Phantom)
Camera/extra camera battery/extra storage cards
Journal/Books to read/Paper, Pen
Binoculars (to check out the big horn sheep, the Anasazi Bridge, the Condors, etc)
All-in-one shampoo (small bottle, safe top, Dr Bronner's is good)
Toothbrush and toothpaste
Hair brush/ties/mirror
washcloth
Anti Fungal and anti bacterial ointment for foot maintenance (a MUST)
Tampons
body lotion, sun screen, chapstick: (lots!)
mole skin (for cracked heels, blisters)
Flashlight/head lamp
Extra batteries
Sunglasses with straps (2-3 cheap pairs, they will be lost/broken/sunk/scratched)
Spare prescription eyewear
Backgammon, cards, ipod, solar or battery re-charger
Library (river book, geology book, star book)
River knife (optional)
Tent
Water Bottles (2), they need a loop/place to attach carabineer
Blanket and Sheet, or Light Sleeping Bag (summer)
Ground Pad (Paco pad supplied by outfitter)
Cot (optional)
Ground tarp
Folding Chair
Day Pack or fanny pack (w/water bottle holders ) (optional)
Personal Hot/cold Cup
Butane lighter (for candles, emergencies, frayed cam strap ends)
Plastic bags: Thick Trash bags (1-3), Ziplock baggies, quart & gallon sizes
Carabineers (3), (personalize them with your own colored tape pattern )
Stash of personal munchies - (ie one cliff bar per day)
One weird, fun, non essential thing no one would expect on this trip, lets have some fun!
Personal Meds (If drug dependent, meds split into 2 supplies and stashed on 2 different boats.) It's a good idea to have run of Cipro for any unforseen problems. Ask your doctor ahead of time.
Small waterproof bag to stuff plane tix/phone/wallet/meds/etc