We've done a variety of things.
One trip, we brought dollar store ponchos. Never again - they were short, sleeveless, and if you have hair they will not stay on your head.
Next trip, we bought Coleman ones (at Big Lots for around $5). Much better quality and kept us dry, but a pain to carry around before the rain and an even bigger pain to carry after the rain.
Last trip, we didn't bring anything and waited until it started raining and then just stopped to buy the Disney ones. A few bucks more, but good quality and we didn't have to hassle with carrying them around before the rain. When it stopped raining, I rolled it up and was able to tie it around my cross body bag, so it wasn't a huge hassle to carry around.