Most of my mental activity is devoted to specific and boring areas. But I do have a few common things that I'm fascinated by.
One of these is what do people actually do when they confront a situation that does not have a pre-programmed response. When someone says "Hi, how are you" we have a response that comes without thought such as "Fine, and you?". It's interesting to think about (and later test) what people do when handed a somewhat more surreal encounter.
I often consider, as a passing fancy, crimes of all sorts. When standing in a bank I amuse myself by planning my getaway in case I rob the place. When I walk past someone leaning against a balcony I consider for just a moment pushing them off.
I regularly ponder politics (of all sorts, not just the set that happens in Washington), belief's, social trends and philosophy.
The more math I take the more I find myself fascinated in passing by simple concepts therein. As an example I once thought it was odd that the only square that can be divided by half to get it's root is four. Then I went on to go through the perfect integer squares and realized you could divide them all by their root to get the root. For a split second I thought I had something genius and then I remembered - "Oh yea, that's how squares work!".
I often consider the themes and ideas in works of fiction of all sorts. Stories can come and go but what I find most interesting is the characters themselves and what makes them tick. This is why I regularly dislike comic books - because the characters motivations are ofen just as far fetched as their abilities. Most recently, I've been considering the Watchmen (both book and movie I suppose at this point). I find it interesting that once you move beyond the most obvious bits about the characters (and how they are essentially somewhat realistic takes on famous super heroes) you find that each character embodies a particular response to crisis. Kovacks sits entrenched in his beliefs and is unwilling to compromise on any point of morality and is destroyed. Viedt confronts the problem without regard to the cost. Manhattan views the problem as impassionately as god and so forth. I think this is perhaps where the real genius of the work is and I find it somewhat shocking to see that Blake's own approach is strikingly similar to my own if carried to their illogical extreme.
There is of course more than that but I seem to spend an inordinate amount of time thinking about the above topics.