I'm going to try to write more. I already have learned a lot by:
* photography. If you spend time thinking about the photographs you take, and use what you discover in that process to change how you take future photographs, you will learn a lot about the world around you. You start to see things that you previously ignored.
* painting / sketching / drawing. Totally different from photography. You are free to represent what you want how you want. Your work can range from highly introspective (can be great while your subconscious brain is solving some other problem) to very literal. I haven't kept up with this because the professional world doesn't look kindly on developers who are splattering ink across a page when they "should" be coding, but it was very helpful in my college years.
* music composition. You think you know something about music? Try to compose some! You will listen to music differently. The deeper you go, the more you learn. Music is an unimaginably vast subject and conventional music theory only gives you access to a tiny fraction of the space that music occupies.
* (audio) Field recording. Have you ever taken the time to listen? To really listen? Probably not. You can get audio recorder apps for your iphone, or use a standalone digital audio recorder. Take these recordings home, and listen to them on decent headphones. Then go back to where you made the recording. Think about how the recording and what you hear are different. We selectively filter out a lot of the world around us, particularly in the audio domain.
* electronics. In support of my audio hobbies, I've built several audio devices, ranging from simple synthesizers up to rhythm machines and modules for larger systems. In the software world, we lose track of the physical world. It is mind-boggling how much effort you have to put in to source parts for a moderately complex project.
and finally:
* Coding. Those of us who code understand software in ways that people who don't do not. It doesn't make it any easier to use other people's software (sometimes harder), but it does change the way you think about things.
TL;DR: Don't just learn stuff, try doing stuff as well.