I had read about the Mountainsmith lumbar pack on these boards a while back .
Independent of that, I went to my trusted home spun camping store and explained my concerns -- WDW trip with a 5 y.o. and my husband and me, that we'd have in it: 3 rain ponchos, snacks, 3 water bottles, sunscreen, maps, a swimsuit or watershoes, etc.
Guess what? The guy handed me the Mountainsmith, the bigger one (there's Tour and Day), Tour, was what he recommended.
He said he bought one reluctantly because he thought fanny packs were "fruity", but it soon became his favorite daypack because it's light and does not hurt his back.
It is on the expensive side. He suggested I load it up and wear it around the home for 6 - 8 hours to see if I'll like it. Apparently my store will accept it for up to a year.
We got a camera case that can attach to the belt or be worn around the neck. We will get a coin purse like that, too, (for keys, cards, and passes). We will get a couple of sunglass cases as well that attach to the belt. That way, there will be little in/on the pack when we stash it various places on the stroller and go for a ride.
The pack can be used as a handbag, shoulder bag, and with an attachment, become a backpack. He showed me how to do that and, frankly told me that the only reason he could see it as being useful was for those people who are still reluctant to use the lumbar belt. He mentioned, however, that it might be used effectively along with the belt so that the goods are more against your back. Even without it, however, the belt does not bend down with weight.
KIS