1. Write a program in the language that I've done before just to get familiar with the tooling. My go to for years was a better Eliza clone. From 1991-2008, I wrote it in AppleSoft Basic, GW Basic,HyperCard, VMS DCL, Visual Basic, C#, PHP, and JavaScript.
2. On the job. I hate hobby side projects. I'm not creative enough to come up with anything interesting and I feel like it's a waste of my time to build something that will never get used. I would rather just work on a proof of concept related to my job after hours and show it during a lunch and learn. I get real feedback from peers and if it's implemented, I get to "productize it" and see it in the real world.
3. On the job (part 2), I'm at a point in my career, where companies will hire me if I only have half of the required experience in the technologies they use because I've worked at a lot of small companies where I've led projects either officially as a team lead or unofficially because the team was so small I could take the lead. I can usually get a job based on what I know well and they give me the opportunity to learn what I don't know while doing a project. I'm more than willing to put the time in to get familiar with a technology I don't know.
4. PluralSight - I'll watch videos to get an overview of a technology usually before I go to work in the morning while working out in my home gym.
5. Certifications - I don't believe most certifications are worth it from a career standpoint and when I'm hiring, I take many of them -especially developer certifications - as a negative signal. I wanted to learn C# in and out in 2008 when I was transitioning from 12 years of C/C++, did a combination of 1-3 and did the highest level of MS developer certification that required 6 tests. I never put the cert on my resume or talked about it during an interview, but it forced me into an organized learning path.
I'm working on my AWS certifications now for the same reason - to force me into an organized learning path. But I will put these on my resume, they seem to carry weight - especially at consulting companies.