I agree with ponchos, but I see lots of people comment on how thin dollar store ones are and to take more than one of them for each of you - we bought our ponchos at Disney (our our rubber ducky yellow, they now have clear ones), they have Mickey on the back, and last year my DS bought an adult one for $6 at one of the parks. We have had ours for four years now, and use them every year - they're thick and comfortable, fold up small enough, and last quite a while - also come in children's size cheaper. If you have to keep buying dollar store ones, it's probably more worth it to buy the Disney ones and keep re-using them.
We go every year last two weeks of August - we love the Florida heat, but definitely take bottled water with you every day. We have a DVC kitchen in our one bedroom, so we always keep bottled water in the freezer then take it with us during the day and drink as it melts, stays very cold.
Hats are a must, and sunscreen too.
Rain doesn't make any difference this time of year to your enjoyment - never lasts long, and each park has lots of indoor shows and rides to keep you dry - most of the line-up areas are covered or inside as well.
No matter what, you'll still enjoy every minute!!!