jeans and capris
A cute tankini, or one piece with a pair of cut off jeans, a tote with a rolled beach towel, extra dry clothes, and cowboy boots..you can use the beach towel to sit on at the concert, the extra dry white shirt to wear over the tankini ( a sleeves cut off a white cotton button down), is what I would use..a bottle of nice smelling tan lotion, don't forget..have fun.
You'll look like everyone else there. Why not find a cute red gingham top and tie it in front, with jean shorts?