One of my favorite (and most relaxing) pastimes is staring off into the sky, so a good portion of my day is usually spent thinking about the contents therein. Stuff like cloud classification, what is causing the color of the clouds, what the color signifies in colloquial terms and if this is congruent with the scientific meaning, and so on.
I also like to imagine fantasy scenarios involving one or more people that I know. For instance, one of my ongoing scenarios involves the invasion of my university area by zombies. Silly, I know, but still really really fun to think about. Especially thinking about how you would get out, and how other people might behave and what you'd have to do with them/what roles each person would assume. And if I'd be okay shooting a close friend if they were infected.
I also like to think about my future, and what I'll be doing/what I'd like to be doing.
Of course, this doesn't capture half of what goes on inside my head on a daily basis, but it's a glimpse. Oh, and I forgot to mention, analyzing people's behavior in order to type them occupies a significant portion of my daily thoughts. It's just fun.