We go every August (5 wks, 6 days, and counting

) and I have a few musts:
big and small ziploc bags (to keep things dry/to keep wet things locked up)
2 pairs of walking shoes--they get wet so easily, plus we all change shoes during our midday break to avoid blisters and such, and water shoes for pool and WP's.
rain jacket instead of poncho which doubles as coverup when cold, plus usually less bulky in bag.
sweatshirt for kids (picture this--it starts raining in parks, we get wet, decide to head for DQ, start freezing as soon as we get there--you get the idea)
lots of socks--again, they get wet so we all change midday
coffee/splenda--what IS that stuff they call coffee in WDW???
lots of meds and extra prescriptions for my son's meds (asthma)
Clorox wipes (once we checked into the boardwalk and started throwing up within hours, found out that the cruise ship flu had invaded the hotel the day before, now we wipe down everything as soon as we enter the room!)
I wear capris on the plane so I have them with me. I usually wear them when we go out for nicer dinners (like Ohana's). It's not necessary but I feel more comfortable.
I get Avon sunscreen with bug repellant so I only have to put one thing on.
We don't do the misty fans anymore. We found them too heavy to carry around when they are full. We freeze water bottles over night and bring those with us instead.
Laundry soap pre-measured in ziplock bags and some bounce/shout sheets. Some palmolive sheets to wash things out. Extra batteries. A familiar toy for my kids to play with in the room. A favorite movie to watch on TV and/or a favorite book to settle down at night. Antibacterial hand gel (for when we want a quick snack and aren't near a restroom to wash).
At least two bathing suits, and lots of swimmer diapers.
If you have trouble with your feet or skin, bring some oatmeal bath stuff with you. One of my kids always seems to get some unidentifiable rash when we're there (who knows why--too much sun, sunscreen, pool chlorine, etc) and it helps him to have the oatmeal bath. I also like it for my feet. I get that weird sock rash everyone always talks about here.
August is really not as bad as you think, as long as you *roll with it* and find something to do when it rains, take a midday break from the parks/crowds/sun, and realize that seeing EVERYTHING in one trip doesn't necessarily make it a good trip. Be flexible with your schedule and just decide to enjoy whatever you end up doing.
Jenny