My mind never stops...it's like a constant internal dialogue that takes in information around me, assess it, and evaluates (or catalogues) it. Simultaneously I can be thinking about any number of things -- school, work, friends, family, things I have to do, the past, present and future, money matters, logistics, food. A lot of my thoughts are mostly sarcasm, puns, and insults...it's sometimes hard to keep a strait face if my thoughts try to run away with me.
I don't know about other ISTJs, but I think I'm a pretty good self-monitor, so I think a lot about my audience and the best ways to present my thoughts to them. I gague my next moves based off their reactions.
My mind works in lists and webs...it's all connected up there, and I am a master with mneumonic (sp?) devices. One thing can trigger a memory which can lead to a chain-reaction of random thoughts. Before I know it, I'm daydreaming. I would be willing to bet that the majority of what I think about has to do with my immediate surroundings...Si at work, perhaps?
I think a lot of times I probably don't realize how quiet I really am. I have all this dialogue going on in my head, but only a small fraction actually comes out as speech.