I bring a little assortment of different sized Ziploc bags. They come in handy for a million things, especially when my son doesn't finish a snack or a meal. When he whines that he's hungry 15 minutes after we left a CS, "Hey, here's that burger you asked for but didn't touch 20 minutes ago at Pecos Bill's, kiddo."
A thermometer. I hope you never need it, but they're hard to come by at WDW if you do.
Liquid hand soap. I don't like washing my hands with bar soap and it's easier for kids.
Cut up a sponge or two into small pieces, wet them, and drizzle the pieces with dish soap. Dry them out completely and store in a Ziploc. When you wet them again you'll have little dish sponges, great for washing mugs or sippy cups or kid utensils.
If your son is still young enough to take baths (not showers) a plastic rinse cup. Some resorts only give you glass and/or tiny plastic cups.
Pennies and quarters for the pressed penny machines, with some sort of pouch to store the pressed ones.
Have fun packing and have a GREAT trip!