Be sure to bring your sewing machine's manual, feet, and power cord.
Extra rotary blades
Iron cleaner, esp. if you are into applique or do a lot of pressing.
The one I go to every year is held in a hotel that puts a fabric table cloth over the tables (the tables are nice, sturdy wood ones - but the wood is kind of rough). Experience has taught me to bring an inexpensive vinyl table cloth to put over the fabric one - makes cleanup easier. One w/ as little pattern as possible so you can see what you are looking for, though! I like to clean up each evening and after lunches.
A chair cushion - the one I bring is pretty thick and in addition to providing comfort for the bottom, it raises me a bit.
Hope you have lots of fun! Please let us know how it goes.