Depends on what you plan to do and if you mind doing laundry!
If you're strictly casual, coordinating shorts, tanks, tees, one pair of long pants, a windbreaker type jacket (A/C can be nippy, and the windbreaker doesn't get soaked too quickly in the rain), 2 pairs of very comfortable shoes (forget fashion, you're going to be walking 2+ miles daily) and undergarments of your choice. Also, prescriptions in their original containers, contact lenses (spare pair)/eyeglasses and related supplies, OTC medications and personal care (aspirin/pain reliever, sunscreen, insect repellent with DEET, etc.) and other items you require for survival.
I take one checked bag and have a small wheeled duffel that I carry on. My carry-on contains two sets of underwear, one change of clothing, a small bottle of Woolite or dish detergent (emergency hand laundry if luggage is lost), eyeglasses and contact lens stuff, small personal care items (toothbrush & toothpaste, travel size shampoo & conditioner, brush, etc.), medications (prescription & OTC), a book and a couple of magazines for the flight (5 hours on Southwest).
Since my WDW trips usually are 9 days (7 days on site, plus a travel day on each end), I take five pairs of slacks (I don't wear shorts), 6 or 7 tops that roll or fold very small, 6 or 7 pairs of socks (wet feet are not fun and socks take quite a while to dry with the humidty), a light cardigan-style jacket, 2 pairs of walking shoes (wear one, pack one) and a pair of Birkenstocks that can go a bit dressy, a long skirt or long tank dress (I like to change from park mode one or two evenings), and necessary underwear. I do laundry about mid-point rather than carry additional clothing. All of the clothing coordinates so I can interchange the pieces to look slightly different. The additional personal care items are packed in my checked bag.
I also take a hat (very fair and burn easily) that can be scrunched into my shouldbag while on rides.