I pack rain ponchos--I figure if I have them , I won't need them! Plus buying them at WDW is expensive, and they're flimsy. Altho, I read if they rip you can exchange them for a new one. I make sure the kids' sunglasses have those holders so they can take them off and have them hang around their necks.
I bring the tiny bottles of hand sanitizer--I figure it's better than nothing when we stop for ice cream or snacks.
We have 3 DSs, so we bring the walkabouts so we can split up and find each other.
I also bring along children's tylenol/motrin and cough/cold medicine. The dissolving tablets don't take up much space, and we've needed them on vacation before. I know you can get some of this onsite, but when your kid wakes up in the middle of the night with a fever, you want something on hand.