I've carried a back pack and a fanny pack. The back pack was for little DS, so it had all his stuff in it, plus extra hand wipes, etc.
This time, I'm thinking I'll only take the back pack, where I'll stash bandaids, Chapstick, hand wipes, Tylenol, extra baggies (for camera coverage in case it rains), sunscreen, film, extra clothes for little DS, some things to keep little DS occupied while waiting in lines, my prescription sun glasses, PS numbers, pad and pencil for trip report notes, maps of parks, cell phone?, and other stuff that I can't seem to remember now, lol. (I need to refer to my master packing list!)
Hope this helps some.