Great question! Right off the bat, when I was considering a major change to engineering I was immediately unsure of which discipline to go into. Some quick research into what type of work each discipline does, not to mention the sub-types within each discipline (for example within Petroleum Engineering there are the subtypes of Drilling, Production, Operations, and Reservoir Engineering) gave me a better idea of what I would like to do. That research, coupled with speaking to people I know that actually work as engineers helped me make my choice.
So that was the first time I learned something outside of college that helped me. Other than that, the most important out-of-college learning that I experienced, hands down, was via my internships. A lot of students when I was still in school didn't try very hard to attain internship positions, instead choosing to use their summer breaks for travel, relaxing, video games, etc. While all those things are fun, you gain exactly zero experience and invaluable knowledge by missing out on internship opportunities.
Once I had changed my major to Petroleum Engineering, I still wasn't sure which sub-type excited me most, so I pursued and attained three separate internship positions over the course of three summers, the first in Production Engineering, the second in Drilling Engineering, and finally the third in Reservoir Engineering. By doing so I not only got a better idea of what I wanted to do after college, I actually got a massive leg-up on my fellow students, learning more outside of the classroom over a couple months than I could ever learn from a text book. The value of hands-on experience, whether in an office or a field setting can't be overstated, regardless of engineering discipline or sub-type.
I hope this answer helps and I didn't ramble too much! Best of luck!
To be clear, there is a whole lot that you college graduates have to learn once they have completed their degree. Having graduated a little while ago, I share that 50% of what I learned after graduation is/was technical and 50% is/was non technical. I also want to caution that college graduation doesn't necessarily mean the end of learning. Many successful engineers understand that learning doesn't end because technology is always changing and growing.
As far as technical learning, I have learned(and continue to learn) new programming languages and new platforms that house data. As for as non technical learning, I continue to learn new ways to problem solve and building on ways to work well with internal and external partners.
I hope you find this guidance helpful. Best of luck to you!
Technical: Matlab, Python, Aspen, STATISTIC,...
Non-Technical: Net-working (Linkedin Profile is a MUST), leadership, volunteering...