I hope you enjoy your retreat. I love them. Two things I haven't seen mentioned are:
Extension cord
The pedal for your sewing machine!!!!! -- You'd be surprised how many times people have forgotten that one.
The rest is stuff you would use to sew at home.
I pack everything into my large suitcase (including my sewing machine), and then my fabric is in another container. I usually cut out my project before I go, mainly because I really need to concentrate when cutting out, and the tables are low.
Wine is always on my list as well.
Hope you have a great time.