I agree with the simple, easy projects that are already cut out. Chocolate, check! No wine, though - we usually end up at church retreat facilities and they don't allow alcohol, which is a bummer.
One year I brought an applique project that I had cut out. I sat with my paintbrush and my starch and got all the edges of every single shape prepared and ready for sewing. It worked out great! It's such a tedious job, but since I was doing it while chatting it went quickly. Another year I brought a simple flannel 9-patch, precut, and had a finished top by the end of the retreat.