Are you going with a friend? If you are, share tools, mats, etc. No sense both of you bringing everything. Are linens supplied? I bring my own jersey bottom sheet (they use flat sheets only). Many of us bring a quilt we're made to "show off our beds". Some bring sleeping bags. Some retreats provide coffee, tea and tap water only. If you have a favorite beverage bring it along. Bring your personal cup. We ALL bring food goodies to share and we even have a happy hour. Always good to have some chocolate on hand. If weather is a factor, dress in layers. Bring a flashlight. I bring my own chair. If I'm going to be sitting a long time I want to be comfortable. If you don't want to bring a chair, bring a cushion or extra pillow. Go with a mindset that you are going to have fun--feel free to ask others about their quilts and don't hesitate to ask for help. You will make lots of friends. You might even want to make it a yearly tradition. I've been going for about 8 yrs.
And I'll say it again - Have a great time! Come home with stories to tell.