Bring a coffee cup (if you drink hot drinks) and a water bottle that will not spill if knocked over. These are 2 things that are top of my list. IF you are sharing tables....you don't want someone's drink spilled on your project. Your own chair.
If you are taking your vehicle I put extra cash in an extra wallet under the seat for safe keeping. Sometimes if there isn't any place to spend money at the retreat....I just leave my wallet in the car. Those are the only things I can think of that weren't mentioned. Have Fun!!!