I think a lot about words and meanings, and interesting things I've encountered. Like, what is the difference between catenation and concatenation? Why is unravel a word, when ravel means the same thing, and given that unravel has taken over the meaning of ravel, why hasn't ravel come to mean it's opposite? Why was the classical Latin pronunciation of "caesar" preserved in German but not English? Why are the common conception of zero as an absence, the scalar zero, and the vector zero all different?
Sometimes I just daydream about stuff. Often I remember things that make me laugh, or make me angry, or both. Occasionally I force myself to actually work on some of the projects I've daydreamed and planned out (forced, because for some reason that part is much less innately interesting to me than the planning and conceptualizing, even though I want to finish them).