all good suggestions so far
I'd like to add, from my packing list last summer...
*sunglasses and umbrellas - if they break or get lost, you're only out $1
*playing cards, puzzle books and/or small toys for entertainment on the trip/during down time away from the parks or by the pool
*caribiner clips (4pk) for clipping hats or travel mugs to belt loops or bags on rides
*all sizes of plastic zippered bags (snack, sandwich, quart and gallon). I use them for almost everything I pack
*glow bracelets/necklaces
*autograph book and/or photo album (if you can find ones you like)
*pen/sharpie - sometimes they have fatter pens suitable for the characters to sign autographs
*mini notebook if you like to keep notes during your trip
*toothbrushes (2-5pk) and dental floss - take along new and throw away before you go home
*clothes pins
*extension cord (make sure it has a UL tag, there were some that were recalled a while back because they were not UL listed)
*travel first aid kit (prepackaged with bandaids, etc)
*magic towels (great to carry in a ziploc bag with ice/water on hot days)
*window clings to decorate you room's window will help you find your room along the corridors of look-alike doors
It all depends on what's available when you shop. I haven't seen pop-up hampers in at least 6-9 months and could really use new ones (without spending $3-$7 elsewhere).
