Bring your lawn chair, there is usually food, but bring a cooler too. You can bring your instruments, just not to the main seating area. Many people will be "pickin" in the parking lot. I have found the smaller the festival, the more opportunity for jamming. Sit on your tailgate with your instruments out and see who joinns in. You can also find someone's to join.
They will let you know who is performing and when, it's almost always an all day thing. Like noon til 10 or so. You will have a blast! Call me if you need any more info!