BarbinLA has pretty much covered everything. My retreat/class machine is a Singer Featherweight. It travels in a Coleman soft-sided cooler that has a plastic inner lining. It also has outside pockets and a top zippered pocket, so I can carry my notions, fabrics and tools. A heavy-duty multiple plug extension cord is a must (mine is one of those that winds the cord up and has 4 outlets -- sewing machine, light, fan and iron. I have a wooden tea tray that I put batting on and made a removable cover for a pressing station and my 12" x 18" cutting mat fits it perfectly. If it is a class-type retreat, they must furnish you a supply list and any advance cutting instructions. If it is a do-your-own-thing retreat, decide on which projects you want to do. I always take more than I know I will get to, so if I change my mind, I have something to work on. Do any cutting at home so that you will have more sewing time at the retreat. The last retreat I went to, I decided to take only UFOs and got 4 quilt tops finished, made significant progress on another, got a little paper-piecing in, too, on an American Beauty kit (this is my "retreat project" and goes to every retreat, and worked on a fusible BOM.
One thing I would suggest is that you find out where the retreat is being held. If it is at a retreat center, they will probably have a website that you can look at. Also, see if there are any LQSs in the area where the retreat is being held and plan to take a couple hours to visit them.
I belong to two quilt guilds, and each one has at least one retreat a year. I enjoy getting away from the "real world" and spending time with the other guild members. It gives me a chance to learn more about those I haven't gotten to know.