Even if the place supplies ironing stations and cutting stations, a lot of the time there is a line to use the equipment. So I take a small cutting board/ironing board combination and a travel iron. I also have a bag with all the basic sewing/quilting supplies. My retreat bag has a plug strip in it just in case. The setup is usually shared and with all our gadgets these days, a single strip doesn't cut it.
The first time I took too many projects, the last time I took too few. One thing that works for me is to pre-plan the projects and pack each one in a project box or bag. That way I have everything at hand when I pull it.
Space is generally limited so you don't want to take a project that takes a lot of space such as quilting a large quilt. It is also best to take something where you don't need a design wall. If the place has design space, great - but many do not or they are taken by other members. I find that I need to work on simple things because I spend A LOT of time visiting which is tons of fun but it does not help with concentration.
Since I have back problems, I generally take my sewing chair to retreats as I never know how good (or crummy) the seating is at the place. If the chairs are good, I may leave my chair in the car.