When I go to the Philadelphia Orchestra's outdoor concerts, I like to wear a knee length, A-line, light weight denim skirt fitted at the waist (no elastic), a cami top and a pair of sandals. I bring along a summer weight sweater if it gets cold. We sit in back in the grassy area on our low beach chairs or on blankets and bring a bottle of wine and picnic basket of food. They have a handful of tables further back and chairs closer to the stage. People are dressed in an assortment of styles, mostly casual though. You might want to put on some type of bug repellent and skip any other fragrance too.
Have fun at the concert. I saw Chicago about 20 years ago when I was 16 years old in Pensacola, FL. They were great! I think I wore jeans and a t-shirt then.