whitesides, I'm a little (well, more than a little
) type A about packing. Here is what I did: I made a list of each day's activities and what I'd want to wear for it, and used that to pack.
So day 1 is meet at airport, walk around Mont. Castle, dinner at hotel. One outfit would do for all of that, and no special shoes are needed.
Day 2 is jeep tour and lunch in a barn, then rest of the afternoon on your own. You do get a little dusty on the jeep tour, so some people changed after that. I was fine with one outfit b/c we went on a serious hike after lunch, so I wore my hiking clothes all day.
Day 3 is a nature walk (HOT) followed by shopping and lunch, then the drive to the GC. No opportunity to change, so plan accordingly. Most people did change for dinner that night. Not to dress up per se, just to get out of what they'd been wearing all day and to feel fresh.
Day 4 is bus to sights at GC, then on your own. Again, one outfit is fine, and you might want a change for a nice dinner or if you are going to do a serious hike.
Day 5, another long bus day. Also, a dusty ride at Mon. Valley, so maybe a change of clothes for dinner. I changed into the same outfit I changed into for dinner at GC, but would have been fine not doing so.
Day 6 is Arches Hike, then afternoon on your own. You will want a hiking outfit and then whatever you need for the afternoon activity. If you ride horses, then jeans. If you do ATV, you could probably keep your hiking clothes on. If you hang out, then swimsuits. That night is adult dinner at the RCL restaurant. I put on a skirt for this just to make DH happy.
Day 7 you need clothes for your morning activity, and your rafting clothes for the afternoon. They suggest a rash guard or tshirt that can get wet for under your life vest, so it won't rub. That night is the dinner and campfire. Don't wear the clothes you plan to wear on the way home, b/c they will smell like a campfire.
And Day 8 you fly home.
I had real hiking boots b/c of our pretrip, so I wore those on the hike, but you definitely do not need them on this trip. A good pair of walking shoes and then a pair of sandals/river shoes is all you need.
Also, I always like to remember that we will be taking tons of pictures and showing them to our friends and family, and trying to find a Christmas card picture in there. So we don't dress like slobs on vacation. The type-A-ness comes in when I try to have all 3 of us be not only neat, but somewhat coordinated with our clothing. DD and I end up wearing the same colors on the same day, and DH pretty much wears his man-i-form of a dark top and tan bottom every single day of his life, so he always goes.
We took a lot of Under Armor tees and ACG tops and shorts (Nike's new line of casual active wear). These look more polished than big tees and baggy shorts, but are super heat-beaters. They don't wrinkle and they pack small, all important in my book!
I did not pack a raincoat. It did end up raining on the day we were supposed to ride horses. I bought a jacket at RCL, which of course made the rain stop before we actually rode.
Doing it again, I think I would have thrown in a tiny disposable poncho, just in case.
HTH! You're going to have a great time!
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I'm sorry I was such a dork about the backpack carryon issue. I just picked them up from my AAA agent's office last week and see now that they will easily fit in our suitcases. I had somehow envisioned them as larger and bulkier. 
