You don't want to not have what you need, but you don't want to lug too much stuff around all day. . .
Think basic weather and necessities
For Sun:
sunglasses
sunscreen
lip protection
For Water:
disposable ponchos (take up very little room)
camping towel (also very little room)
trashbag (to put your pack in on water rides)
Food:
you probably won't need but one small snack per person (e.g. two packs of crackers) maybe bottled water, but can be heavy
Medical:
bandaids, tums, bonine, immodium, tylenol, mole skin
And don't forget your Camera
the worst mistake to make is carrying way too much junk. . . much better off to forget something and have to spend 5-10 bucks than have a sore back at the end of the day. trust me I've done it both ways!