I've been on tons of retreats; they are so much fun! Some are structured (have classes and events), some are completely unstructured. No matter which one you go on, you will have a blast and make many new friends!
I created a list in Word software just for items to bring to retreats. The most important things (for me) are Ott lights and power strips. There never seems to be enough lights and since we quilters have so many things that require electricity, power strips are an absolute must. Depending on your own height, you may want to bring a pillow to sit on in the main sewing room. If you're using a regular chair and the tables they provide, the table might be too high. If you use a chair on wheels at home and can bring that, that would be very helpful to you.
Don't forget your clothes and toiletries - you just might be focused so much on your sewing stuff that you could easily forget those!
Some retreats I've been on have been at a great hotel on Cape Cod. We take both their conference rooms. All of our sewing stuff goes in one room and in the other room are tables for all the snack food, several tables with bed risers on them, then 2-3 tables put together with chairs around it so people can eat together. We even set up Crock-pots for dinners since this retreat doesn't include lunch or dinner.
Oh such fun, wish I was going on one right now!!!