Do you really need a backpack? If you have little ones and a stroller, I can understand this - but if not - cargo pockets may be the way to go!
We always used to use a good sized fanny pack - but I hate waiting in those long lines at opening for the bag check.
I now use a bottle strap for my Brita bottle, and a Passporter Pouch (wallet type thing on a string around my neck) for my park passes, pen, money, etc. DH wears cargo shorts, and he has room in the pockets for our small digital camera, a couple of disposable ponchos, etc. We can go through the parks like this from opening till closing - no wait, and no worry about losing things!!
Of course, we do stay on property, so getting back to the resort to change clothes, etc, is just a quick monorail or bus ride away.
I don't know why Disney bothers - you can fit just as much into those cargo pockets or more than a small fanny pack! But we've been told repeatedly - even the Pouch is not a bag (even though it is almost as big as a small fanny pack - if it has a belt, it's a bag, if it has a lanyard, it's a wallet!)