I'm off to a retreat this afternoon. For this one there will be an actual 2 day class with Shelly Burge. She sent out a supply list needed for class. The retreat center sent out a list of things that are furnished and what is not. At this particular center, they furnish a snack room (price included with registration) has coffee, tea, soft drinks, water, micro popcorn, package snacks, yogurt, apples/oranges/bananas, even chocolate chip cookies.
You should get a list of supplies from the organizers. One I attend, we have to take our own bedding and towels etc. Ask the retreat sponsor.
For your own sewing, your pattern, fabric, threads, cutters, mats, rulers, sewing machine and all you need for sewing your project, extension cords, possibly iron and small pressing surface, sometimes large board and iron are furnished.
Clothes, something comfortable, most retreats are NOT fashion statements. You will be sewing for long stretches of time, so think lots of water to keep hydrated.
Most of all go with a smile and have a ball.