I spend a lot of time thinking about the human mind and behavior, but I always enjoy learning things, no matter the subject. As a consequence, people with a passionate love of knowledge for a given subject tend to attract my attention.
I'm also fond of the outdoors, with a particular fondness for hiking and mountains. Really, anything that gives me a chance to escape into nature.
I typically read nonfiction, so it's hard to come up with a favorite: it all gives me thought-fodder. Depends on the month's fancy.
In terms of fiction, when I rarely find the time for it I opt for sci-fi.
More specifically, I spend a lot of time thinking about meaning. Language, information, life - all fall under that heading, and I like to think about how it relates to how the mind works, and what it's for.
Lately questions of causality have been on my mind, what with it being completely and utterly impossible to prove.