- Exercise: Running and bodyweight strength training are free (minus clothing), and can be done more or less anywhere. They yield clear results as you get in better shape, and can be really fun when combined with music that pumps you up, letting it drive you towards runner's high. Some take this further than others [0].
- Music: Listening to a variety of music, as well as performing (preferrably multiple instruments) gives you a greater understanding of what is going on and why it works, though practice can be very time-consuming - but ultimately worth it. There are, for instance, very few things in my life that I remember as equally rewarding as trying to learn Metallica records by ear in my early teens. As your technical proficiency and understanding improves, you may gain an interest in complex and/or obscure and/or straight up weird music. Some take this further than others [1]. In addition, experimenting with recording and manipulating/mixing/mastering will give you an understanding of the production-side as well. Knowledge of both performance and production will affect the way you hear recorded music, ideally culminating in an approximate understanding of how the different sounds were made and molded. Combining writing, performing, recording and manipulating - making a piece of recorded music entirely by yourself - can be pretty difficult, but also extremely gratifying.
- Reading literature: A great way to go on an adventure, relate to someone, or see things from someone else's perspective. For instance, I've learned things from being exposed to opinions that I didn't necessarily agree with, like Philip K. Dick's views on abortion, Dostoyevsky's orthodoxy, and various positions of the oft-misunderstood/misconstrued Nietzsche. There are some really great books out there dealing with everything between ordinary people in ordinary circumstances and extraordinary supermen in cooky sci-fi universes. Some take this further than others [2].
- Programming: Picking up and learning some new language or piece of technology can often be rewarding in and of itself, and lead to an added perspective in future tasks. Additionally, programming has the advantage of often giving (more or less) clear feedback. As I'm sure we can all agree upon, no one has ever gone off the deep end with regard to programming.
[0]: https://youtu.be/hKrg_ei96xk?t=88
[1]: https://www.youtube.com/watch?v=J4rqXqpLCZ0
[2]: http://www.amazon.co.uk/Ayn-Rand/e/B000APYGIW/ref=dp_byline_...