Be prepared to get ponchos. I used to think that getting the cheapie dollar store ponchos and bring those, but since we got the cute clear ponchos with Mickey on the back, and since I've used mine for 3 trips so far, I'm all in favor of them.
If you like umbrellas, be prepared with those, too.
If it does rain, the parks will be very empty. Which means you get it almost all to yourself, if you stay!