If I had to describe my mental world, I'd say I'm almost always thinking about something. The only times I'm not thinking about something is if I try to not think about something. Or if I am too exhausted, or have drunk alcohol, but even then I have not drunk alcohol in a very long time and my sleep is good. So my thoughts are roped together, like one string attached to another, one always leading to the next. It's usually like an inner monologue, but sometimes I can think in images or concepts too, or think in scenarios and events. Picturing the future or remembering the past comes easy in my mind, as does imagining new and wildly different things. I find it funny however that I can picture very complex things very easily, but sometimes fail to grasp very simple things. All of my friends for example cannot work in their bedrooms or kitchens; they tell me it is just impossible, because they cannot get into the right mindset. But I cannot understand outside my own mindset; to me, a room is a room, any meaning we add to it is painted from our minds. So we should be able to unpaint it too, no?
I get many such questions in my mental world. Perhaps this is not what you mean by mental world, but in my mental world I am always examining myself and questioning my ideas. This process easily takes up the majority of my thinking, if I am not thinking of a new story or problem. I think it is a useful trait to have, but I did not deliberately try to shape this kind of mental thought process, it has just always been this way for as long as I can remember.
Probably the most interesting thing for me is that the voice my inner monologue uses - has my accent, but does not have my voice. It may be a voice I used to sound like, but it definitely is not the voice I have now. I hear many people can change the voice of their inner monologue too, so they can re-read this with Morgan Freeman or David Attenborough's voice. This is much the same for me, but if I stop thinking about it, it quickly returns to this "standard thought voice."