The first, is the past for me too. The Life I have lead till now. Certain people and experiences come to mind that might make me feel pretty nostalgic или the exact opposite.
The second, has to do with my future. Various possibilities on how it could develop пересекать, крест my mind.
The third, is Fiction. Sometimes it might be getting mixed with reality as well. Mostly make-believe scenarios about my Life, Series that I am a Фан of или simply the World in general. There are no limits here so it consumes the biggest amount of my time as well, I believe. It is quite easy to get Остаться в живых in its vastness.
All those three contribute heavily into getting quite Philosophical at that moment !!!!
65% of time my thoughts go into analyzing what I`ve done and what I should improve, in terms of health, lifestyle and в общем и целом, общая to become the person that I`d really want to be. 25% goes of nostalgic, happy moments. Then there are 5% of 'oh god, why' moments. And 5% of something else, unrelated of anything. ^_^
(I`m not mentioning those times when I think about different plot points and twists to any of the stories I`m writing, because I`m counting that as something I deliberately plan on doing beforehand)
I daydream. I kid Ты not, I daydream so much I daydream in my actual dreams. I could be dreaming about being able to fly and I'm still Мечты about the latest fanfiction I read in my dream.
I also like to rank/organize everything I like. Избранное bands, Избранное manga, Избранное tv shows, etc. If I don't know the order I just alphabetize them in my head. I do this on a daily basis.
When I do something particularly weird/embarrassing and I have time to myself, I also try re-enact other people confronting me about сказал(-а) situation and me explaining myself, so if someone does try to talk to me about it later I know exactly what to say.