I read every post, lots of great advice here. Having been on many retreats myself, I created a document in Word software of things to bring. If you're not as comfy creating a list on the computer, start a list in a notebook (you can use the notebook at retreat for people's names/addresses, recipe sharing, quilt pattern idea sharing, etc.)
I bring my own chair to retreats. I'm used to sewing on a chair that has wheels, so I got a cheap small chair to take to retreats with me. Makes a big difference.
Bring snacks/goodies to share with everyone. We usually have breakfast and dinner, and then "graze" at the goodie table for lunch.
Bring your Ott light. Once the sun goes down, you can't be sure there will be enough light for you to sew with.
Cutting your projects ahead of time is more helpful than you think. I tend to bring all my fabrics with me and cut there. The problem with that is that I am bringing so much extra fabric and it all has to be lugged back and forth.
Get a large suitcase to put your sewing projects/tools in. This works great because you can put only what you need on your table at retreat and store the suitcase underneath. If you need something else you brought, just get it out of the suitcase. The biggest help with the suitcase is that it's on wheels.
Check out the weather ahead of time so you can bring the right clothes. If you're going to sit near a window, you might want a sweater for after the sun goes down. Don't forget any medicines you take.
Depending on where we go for retreat, if the sleeping rooms are going to be visible to everyone, we bring a quilt to put on the bed. Makes for great conversation and makes the bed look nicer as well.
More than anything else, have a great time!