First check to see what things will be provided, such as bedding, towels, lights, tables pots and pans, utensils fridge or freezer etc.
If you have everything cut out and ready to sew, it works out great, but take extra fabric anyway. A cutting board, rotary cutter, scissors, seam ripper, needles thread,glue, pins glasses, plenty of bobbins, brush for cleaning lint, oil for bobbin hook, and rulers. Power cords, surge protector, and make sure power cord and foot peddle are packed. Iron, small padded board for pressing. Some retreats will have cutting table and ironing table for everyone to use. I hope this helps, I am going to a retreat next week, and I'm stressing out what to do ahead to prepare also, so Ill be watching to see if I have forgotten anything.
Treats or small gifts for the other people going to retreat, may want to lay one on their pillow during the day, and wa la
they love it.