I think my answer depends on whether you mean practical or sentimental. For practical sake, I would bring a survival guide of some sorts. How to make shelter, find food, make weapons and other gadgets, how to use your surroundings to ask for help, and so on. Someone might already bring one, but I will just have to be careful not to bring the same book.
Sentimental, perhaps my needlepoint box. Since it is just one week, I would be quite happy to use the time to finish some of the patterns. It seems that I keep starting beautiful patterns, but never manage to finish them.