Water bottles with insulated covers, with a strap. Not really fun to carry, but not too bad. Filled up with ice and water before leaving room (our last visit, can't vouch for now, you can get a free cup of ice in parks), so you can fill up throughout day. Disney water doesn't taste so good, so if you can't get past the taste, get the drink mixes for single water bottles, those are samll and easy to carry. Last ones I bought, Wal-Mart under $4 each.
SUNSCREEN!!! So important, on all type of weather days.
If you carry a backpack or fannypack and have any valuables, put them in ziplock baggies for sure. Water rides and rain......
Mini first aid kit A MUST....Tylenol, a few sinus/allergy pills, chewable Pepto pills, couple of band aids, doesn't take up much room, but can come in handy and SAVE $$$$$.
Me, personally, I hate ponchos, we tried a couple of times and find them more uncomrtable than comfortable. Not sure if from living in FLorida we are use to being rained on at any time. So, I never carry those anymore, but that's us.