I will wear short skorts, shorts, or sundresses.
I ALWAYS wear a straw cowboy hat. Keeps the sun off my face, top of my head, and my shoulders and ears and I look sexy

.
Some things I avoid:
Spaghetti Strap tops-your shoulders will fry no matter how much sunscreen you put on them. Do the sleeveless or short sleeved instead.
white tops-if you get wet on a ride or rained on it's see-through.
open toed shoes-your feet get stepped on and the sun beating down on the tops of your feet will fry them.
cornrows-you'll get sunburned on the new parts and wish you were dead. It's especially awesome when your scalp starts peeling and you still have the cornrows in. Gross.
Jeans. I only wear them at Disney in the wintertime, and only ones with spandex in them because the classic jeans are just not that comfortable for all the moving around you do in Disney.
DH always wears cargo shorts because we go bagless and I shove everything, including an empty backpack, into the shorts to get through the no-bag entrance. Then we pull everything out of his pockets, put it in the bag, and we're off. He always wears a hat, too-baseball caps are useless because your ears and neck will still get fried-he has a battered old Indiana Jones on safari type hat he bought at the MK near the Aladdin ride. He loves that thing!
I also never wear a swimsuit into Disney under my clothes, I've found it ends up being very, very uncomfortable by the end of the day-it chafes against your regular clothes.
Oh yeah, and for those of you who like to wear oversized t shirts with leggings or stretch pants underneath them, be aware that when you sweat, a favorite place for sweat to start is right at the center of your heinie. I can't tell you how many people I've walked behind in t-shirts and spotty sweaty stretch pants where they thought the t-shirt would cover their butt, but when they leaned forward to push a stroller or raised their arms for any reason-

. Not a good look for hot weather, people.