We just returned from 10 days at WDW with our 2.5 yo dd, almost 5 yo ds, and 12 yo ds. The first time I packed our back with the "bare essentials," we could barely lift it. Clearly, this wasn't going to work. After reevaluating, we took some trial-size packets of sunscreen for emergencies (We really coated the kids, especially our 2 redheads, prior to leaving our hotel rooms. Knowing that we'd be returning mid-day, we didn't worry about taking a big bottle of sunscreen. It worked -- no one got burned!!), a couple of pull-ups in case our little had an accident (never did -- but it provided piece of mind for us), a complete change of clothes for each of the little ones (DEFINITELY used these. Between the water play areas and meals, the kids were constantly a mess!), and a small container of wipes.
Things that didn't work for us: Washcloth in a bag. Bag came open some how and the washcloth got everything damp. Disposable sippy cups -- The tops come off very, very easily. Also, the tops don't have stoppers; so they leak. Our little ones did better with bottles of water and sodas in the regular counter containers. Our kids never wanted the snacks we brought from home. I can't say I blame them. The stuff at WDW always smells and looks so much more appealing. Hand held video games--at home, our kids our glued to them. At WDW, they never came out of their travel bags. (Yea!!) Mini-magnadoodle stayed in the car. Our kids enjoyed looking at all the different people in line with them. They never grew bored during a wait.
We didn't have to put our mister fan in the bag because one of the kids always wanted to carry it.