At the risk of being unladylike....with all the heat and humidity - we ended up showering at night after a hot sweaty day at the park and extra clean undies came in handy. Also, a hankerchief or bandana is good for drying sweaty brows or running under cold water and wiping down grimey sweaty faces. Neither take up much space and contribute greatly to comfort levels in the heat.
Raingear. I opted for the umbrella looped through the belt of the water bottle/fanny pack. Good for rain or creating shade - although my brother, the Disney veteran swears by ponchos.
Moleskin and a small pair of sissors - for those inevitable places on the feet where shoes rub.