It sounds weird, but packing for Disney was pretty tricky. When we went to England a couple of years ago, I was determined to pack everything in a carry on, and so I carefully coordinated everything to minimize how much I had to take (see that post here: Packing Light: Ten Days in England).
The trickiness comes in because you will basically need two outfits for each day: not amenable to traveling light! However, I knew that walking around the park would mean being hot and sweaty and I would want to change before going out in the evening.
For day, I chose basic T shirts, including some I usually use for running which are great at wicking away moisture. Running shorts, sneakers, short athletic socks, and a visor finished things off. I carried my Ameribag, which is designed to feel like it weighs less than it actually does. I carried this because I knew I wanted to have my camera and water bottle in the park (hydration is crucial), but next time I think I would try to find a way to get away with less. Actually, who am I kidding--next time I'll have children and will probably be the resident porter!
After spending the morning and lunchtime in the parks, we would usually come back to the hotel for a swim, nap, and shower before heading back out for evening activities. Disney is super-casual, so I didn't worry about choosing anything too dressy: just khaki shorts and a white skirt that could be paired with any of the tops. We wouldn't be walking nearly as much, so the tennis shoes and flip-flops were fine.
The extra items: I brought yoga pants and extra T shirts to wear on our travel days. We rented a fifteen passenger van (there were 8 of us going--6 adults, a teenager, and a toddler--plus all our luggage), and this worked out well. Comfort is key on a nine hour drive. I also packed the necessities: pajamas, swimsuits, and a cover-up.
Here is everything (minus toiletries) as it went into the bag (a large duffle). I liked rolling the clothes together (sorted by type: tops, shorts, bottoms, swim, etc), because they take up less room, and don't get quite as wrinkled. However, since then, I have purchased packing cubes, which I love. They should feature in an upcoming packing post.
Here is a demonstration of the stack and roll method, although I am pretty sure it doesn't need explanation.
I was pretty happy with the clothes I brought--I was comfortable and appropriately dressed and even though I brought two outfits for each day, they didn't really take up that much room.