With the Florida weather and hurricane season, I would definitely pack those infamous Disney Ponchos as well! You never know when a heavy afternoon rainstorm will hit!
In our backpack, with padded straps and a padded back, we carried the bare neccessities: fans, sunscreen, disposable hand wipes and a VERY small first aid kit (it was 99 cents at Target). The padded straps help and when my back got hot, my wonderful DH carried it!
Also, don't forget moleskin pre-cut into sizes for your toes, sides of foot, heel, etc. Moleskin saved my little piggy on the last trip!
We rented a locker on days when we wanted to carry a little more into the parks.