The first two years we also rented a stroller each day so the stroller helped us lug items around. It was wonderful but the last two years the kids have been able to walk more and we haven't needed the stroller.
This year Dad looked like this:
The front pocket held a few band-aids, hand sanitizer, a map or two, and other little items like extra pin backs. The big pockets held our jackets, and on this day, our rain ponchos and jackets, a sweater or sweatshirt, sometimes a pair of pants. Throughout the day we take off jackets and put them in the back pack or put jackets back on later in the day. This new backpack was great because it also had the outside straps. The straps could be cinched up to make the pack skinnier when we all had our jackets on or expanded to accommodate the extra stuff in there. The straps were also great for hanging our cups and on the day it rained we hung a wet towel on the strap to dry out.
This was a great pack! I have to tell you though that before we left for Disney Dad asked me what I though about getting a new pack and I told him that I didn't think it was necessary. He went ahead and bought it anyway and I am glad he did. It was a much better pack than the other 'book bag' types we had used in the past. The weight distribution was so much better an on the front there was an extra strap to hold the shoulder straps together which really helped distribute the weight and give it support as you were wearing it.