* medicines: expect the worst, hope for the best. Make sure you have both adult and kid types of pain relievers (Tylenol, ibuprofen), tummy stuff, allergy stuff (always good to have Benadryl on hand with kids anyway) and any necessary prescription deals. Chewables are easy to carry in a Ziploc bag with other meds. Cold meds to hopefully leave in the bag.
* speaking of which, Ziploc bags. Gallon, quart, sandwich, etc. Store half-eaten snacks, meds, first aid stuff, things you want to protect from water, water-logged things you want to keep from other stuff, etc.
* first aid stuff -- polysporin, bandaids, precut moleskin
* sunscreen, hats, sunglasses
* easier shoes for pool time
* stuff for the plane to entertain the kids -- books, movies, drawing/coloring stuff
* chargers for electronics, including portable ones
* autograph stuff