I usually bring a simple knitting project too to give myself a break from my sewing projects. Something I can work on but still chat. In my experience, retreats are very social times and I can't have something I'll need to concentrate to complete.
One thing I would recommend that people don't do is play music. Not everyone likes to sew with music on and in a large group, not everyone will feel comfortable being honest when asked "Do you mind?". Bring earbuds if you need music. Be a good neighbor and use the space you've been allotted. Go with an open mind and have fun!