I just looked up your retreat.....wow! If I only lived a bit closer!
That being said, I go on 3-4 retreats a year and love them all. It is a great time to share ideas with others and get in a lot of sewing time.
You have been given great ideas by others so I'm echoing some and perhaps adding a few extra. Be sure to take a light/lamp, power strip, chair cushion, extra rotary blades, rulers in several sizes, machine power cord and pedal (these are often forgotten), several precut projects, small iron and pressing board, small cutting mat, thread in several
neutral colors in case you need it for the retreat project, bobbins already wound, machine feet, thread stand if you use cones, seam ripper(!).... A rolling tote for your machine and also a rolling crate for everything else is a nice help if you have them.
Label everything! Rulers, cords, scissors, rotary cutter, etc. Also take a sharpie in case you purchase something new.
Have fun! We'll all be looking forward to a follow up post! Happy early birthday!