I like to wear a sleeveless or short sleeved knit cotton shirt, skorts (but also sometimes opt for either walking shorts or capris), a visor, sunglasses, and comfy walking shoes/sandals (usually my TEVAs--but I do try to alternate different styles of footwear throughout our WDW vacation, in order to help avoid any chance of blistering). Also, I use plenty of sunscreen at all times.
Last year I did wear a sleeveless sweater top and a medium length skirt with comfortable sandals for one evening out to dinner and the F&W Fest at Epcot... and that was very comfy and not a problem at all. I don't think I'd want to try dressing like that if we were planning to do a lot of 'thrill rides', or even for spending an entire day at MK or DAK. But it was a nice change of pace from typical park attire, was fine for touring around World Showcase, and really looked nice for our photopass photos at Epcot that evening.