While an external frame backpack is a wonderful thing to have (and I have one, a Kelty), it is not really very suitable for carrying onto theme park rides, as once you are seated on the ride, there is no place to put it. For ride lines, something small and soft that you can cram into a diaper bag is a better choice.
I have one specially made for the purpose, (consignment store, $2) but any large fanny pack will do as a kid prop. The thing to do is to cut a block of styrofoam to fill the interior of the pack. The child sits on the pack, taking the weight off the adult's arms, and transferring it to the adult's back or hips. Still not the most comfy thing in the world, but at least that way you can relax your grip and stay a little cooler.
An unpadded fabric sling should work very well as a hip carrier. The child will still have to hold onto the adult, but not as tightly. I'd suggest something like a Maya wrap, made in light-colored cotton. With an older child, you can stuff the sling tail up under the rings to pad your shoulder a bit more.
The KangarooKorner site has a big comparison page that discusses the uses of the various types of slings on the market (including their own):
http://www.kangarookorner.com/page28b.html