I keep a list of everything I plan to take and check it off ... then when getting ready to return home all I have to do is recheck my list. All my gear is labeled with my name.
I take a surge protector type of extension cord.
While many may bring irons, please check with the site. Irons do draw a lot of electricity and there may not be enough for too many irons. Also, the more irons there are the more risk of something or someone being burned by accident.
I definitely do NOT take my best cutting mat. Did it once and everyone else used it and it was really chewed up at the end of the retreat. Take the smallest one you can get away with. Likewise I take the least amount of rulers I can get by with and, yes, sometimes I share with my best quilting buddy.
If possible cut your pieces before you go. I've contacted the different gals in charge of retreats and have been granted the access to this as long as I promise not to share!!! I do not cut well with others as I am too easily distracted.
I now know to take Aurifil thread and have my bobbins prewound. More of this thread goes on the bobbin that anything else I would use.
If you can fit a camera in it would be great to document the retreat for your memory file on your computer.