Honestly, and maybe you're already planning to do this, but a backpack won't be enough. We rented a Bob for the days we were there and brought a small soft cooler as well. It's a lot of walking and the Bob can do two kids in a pinch, if they tolerate it. Otherwise I'd shoot for something in the 2k cu-in range. I bought my wife a nice Mountain Hardware pack but there are lots that would fit the bill and probably be less expensive.