I went to my first retreat recently. All of the lists in this thread are good...just keep in mind how much room each of you will have available for working and for "stuff."
I took extra rotary cutter blades 'cause I was doing both paper piecing and regular cutting. Another member didn't bring blades, dropped her cutter and chipped the blade on the tile flooring. Fortunately, my extra blades fit her cutter.
I was told to bring "plenty of projects" but ended up taking too much. But, better too much than running out!