Mostly nonfiction: philosophy (particularly the perennial tradition: Huxley, Watts), spirituality and religion, history, and science (e.g psychopharmacology). I also read old science fiction, classical fiction, and technical programming books.
I go out every day and do a full day-long hike once a week in the Cantabrian mountains. I enjoy camping and sleeping in open shepherd's huts. I hike both solo and with my girlfriend, and almost always with my 14-year-old dog.
Outside of projects, I enjoy writing toy interpreters and learning programming languages with different paradigms: Lisp, OCaml, Elixir. Lisp in particular changed how I think. Discovering homoiconicity and the idea that code is data is something I keep finding relevant even in languages that do not support it.
I enjoy simple practical chemistry experiments at home. Simple processes like alkaloid extractions, nothing elaborate.