This sounds like such a wonderful natural setting for an opera.  Just imagine sitting on your blanket in the beautiful Divoká Šárka preserve surrounded by family and friends while you enjoy the music.  The performance this year will be Rusalka by Antonín Dvořák so try not to miss it.  If you are familiar with the story of Šárka and her tragic love, you will find this a very appropriate setting for the opera Rusalka.