Enjoy your retreat! My guild retreat is in May so I have awhile to wait. It will be my second one. I am not sure what I will take to work on but agree with everyone to take a couple of things so if you are in the mood to do something different you can.
Suggest:
Extra light for your work area
Plug strip or extension cord
Flannel sheet and some command strip hooks to make a design wall if you are laying out a quilt
I try to eat healthier so some snacks to nibble on and favorite beverage
Comfy shoes and clothes
Budget cash or CC limit to hit some LQS or other shopping
Sewing machine needles