Beach towels (it was too cold to need them)
Extra shoes/sandals in case main pair was causing blisters or feet were hurting. I have various foot problems, so I'm not sorry I brought my extras for just in case. Really didn't need the extra shoes/sandals for the kids, though.
Water shoes
Extra bathing suits (it was too cold to swim for most of our trip)
Extra swim shirts
Too many shorts; too many sleeveless shirts (it was cold while we were there)
Sand toys (got a bucket and shovel from Hurricane Hanna's)
Drinks (we drove, so brought bottled water, gatorade, wine, and beer, but we stayed concierge so really didn't need to bring drinks. We'd never stayed concierge, though, so didn't know what to expect. The only thing that came in handy was bringing a bottle or 2 of water into the parks).
Cooler (we had no need for it while at WDW, but liked it for the ride to and from WDW)
Extra sunscreen (just had too much)
Raincoats and ponchos (it didn't rain)
Very glad we had: enough socks (almost ran out); underwear (same); power strip for charging electronics; extra ballcaps; fleece jackets; long pants.