Do it! I am in complete agreement with everyone who has pointed out that you'll be doing a lot of walking in heat and humidity. Athletic wear is designed to wick moisture. It's perfect for long days at the parks. Will you be the height of fashion? Probably not. You'll be comfortable, though! You can always change into something nicer if you have a dinner reservation. Outside of Disney, I am the type of person who only wears workout gear for workouts. I wear it at Disney because it's the perfect place for it. And as others have pointed out, I'm on vacation, who cares how I'm dressed? LOL. I do change my clothes before we head back out after our afternoon swim/rest. I put on something a bit nicer. Usually by then it's cooler and I feel ok in my regular street clothes. The one exception is summer when I wear nothing but athletic wear because it's just way too for me to feel comfortable in anything else.