We usually travel to WDW in the winter, so we always have a change of clothes available because we take the first available flight in the morning which means leaving the house around 3:30ish in the morning and being dressed accordingly. When we get to the resort, the weather is MUCH nicer so we need a change of clothes just to avoid sweating buckets while wearing jeans and sweatshirts.
Layers.
If at all possible, we try to dress in layers. Home to airport to plane is nearly all indoors, so for WDW trips, we try to keep to hoodies paired with a t-shirt. As soon as we land, the hoodie comes off.
Last winter, we bought something called a 'packable parka," that reflects one's own body heat back at the wearer. I was skeptical, as they look pretty thin, but they kept us very warm outdoors in 23 degrees F weather (not FL). Because they are puffers, they easily compact. Unpack, and they naturally puff right back up, no wrinkles. Well worth the $. Some are $$$, but ours were only about $45.
For summer, we have the raincoat version. The summer/spring/fall version packs down into one of the jacket's pockets (sandwich bag size). They double as windbreakers. In a pinch they can be paired with a hoodie, and that works down to about the mid 40's. (FL winter weather). I should add, they are super 90% of the time. Once in a while, FL gets days that are absolute drenching rain for hours. For a day like that, the plastic ponchos are better. We bring a set as back up, but haven't had to use any for several years.