Invest in a Mountainsmith Lumbar Day Pack with Strapettes. It is small like a fanny pack, will hold EVERTHING you carry into the park, and the weight is distributed to your shoulders. To give you an idea of what the bag can handle, here is what I carry with me into the parks:
Digital Camera w/ extra batteries
DVD Video Camera w/ extra DVD's, battery etc
2 ponchos
small first aid kit w/ ibuprofen, bandaids, etc.
two 24oz bottles of water
lightweight tripod for camcorder
small bottle of hand sanitizer
Paktowel
small sealable wet wipes
I suffer with back problems, and this pack is the only one I have ever used that doesn't kill my back. Plus, you don't have to take it off at most attractions, just sit forward.
Hope this helps!!