Our group has frequent Stitch Camps, what gets forgotten the most is a good heavy duty extension cord with multiple outlets, and amazingly the sewing machine power cord if they bring the machine they use at home. Another handy item is a plastic grocery store bag that you can tape to your table for the bits of thread, fabric that gets snipped off.
Have fun, I love Stitch Camp.
If you think you'll be going often the folding wagons sold at Sam's, etc are fantastic, I fill it up at the car, wheel it to my table & go back for 2nd load, so difficult to pack light.