I'm picking up all sorts of awesome ideas from this thread in the NaNoWriMo forums -- it's a 30-step plan for world-building for writers of speculative fiction.
It also turns out that I was mistaken in my belief that NaNoWriMo planning could not start before Nov. 1. The Site FAQ says: "Outlines and plot notes are very much encouraged, and can be started months ahead of the actual novel-writing adventure." This is good. It means that I can put in a little extra effort on Halloween building up the main character so I'm not stumbling straight out of the gate.