I always pack light.
I bring with me my various cards: DVC, AP, Driver's License.
I also bring a small bottle of hand sanitizer.
My phone, of course.
All of this goes in my pants pockets.
With this set-up, I can go through the "no bag" line coming in to the parks which saves time and is less hassle. Also, not using a bag means that I have less back strain, an easier time walking around, and I stay cooler because I don't have a bag resting against my body.
I don't need a water bottle because you can get free water in the park, or buy a bottle if you really need/want to and then recycle it when you're done.
Ponchos and umbrella can be purchased in most of the larger gift stores. They often keep them behind the counter and out of site, but just ask at the cash register.
I don't bring any pain relievers or headache meds because they also sell those in the larger gift shops, behind the counter, like the umbrellas and ponchos.