Sometimes it can get a little cool, since Ravinia is near the lake, so a sweater or light jacket could come in handy.   Other optional items:  supplemental food, drink, water since we'll be out there for a long time; lawn chair (they're also available for rent) or blanket; citronella candle; and, sunscreen.  

What's in my back pocket to deal with rain? Clear plastic shower curtains!  Great for covering food, for a water barrier between your blanket and the ground and for holding over your head and chair during a rain shower!

I'll update this as more things occur to me!