There are contemporary performers, Broadway, ethnic and classical music, to name of few. Feel free to bring candles, a blanket or chairs, wine, cheese and crackers and anything else you would like.
Many people seem to have multiple courses for their elaborate picnic.
You are welcome to bring coolers and picnic baskets into the Ravinia grounds. From the lawn you can hear the music since they have a speaker system, however you will not be able to see the performers. Ravinia is a great summer activity, and I suggest you take advantage of it. Sitting in the lawn and planning a picnic can be a nice romantic evening or a fun night for friends. And since Ravinia is the summer home of the Chicago Symphony Orchestra, the music is usually top-notch.