My Mountainsmith 'Day' Fanny is one of my favorite bags and is my bag of choice for WDW.
Usually it is just my DD (age 8) and I heading to WDW and that bag is large enough for typical day stuff- snacks, water, light jacket, camera. I love the versitility to carry it as a shoulder bag or around my waist. When it is around my waist, it really snugs into the small of my back (lumbar region) and doesn't budge and it is easy just to loosen the waist strap slightly and swing it around to the front when I go on rides.
Which one did you order?
I've had mine for years and it has held up well.