I like to take something that is not too hard, lots of strip piecing or lots of the same blocks. SOmetimes I will bring something that I need some help with. Lots of brains to pick.
Bring more than one project incse you get tired of the one you're working on. Also I always bring 2 machines. If one breaks on day one of a 4 day retreat you're going to be very bored.
I make sure to take advantage of the pool and hot tub. Don't forget a cushion for your chair, they are never the right height.
Some ladies in my guild bring thier own chairs and an extra table, some even bring thier own prtable sewing table. I don't go that far, too much to pack and unpack. A rolling suitcase makes it easy to haul all your projects.