Ask HN: What professional development for engineers have you found effective?

I'm looking to organize some group professional development activities for fellow engineers at my workplace. The general goal is to maintain a "culture of engineering" and continuous learning. I was also thinking that active learning activities might be more effective and interesting to participate in than the standard "Lunch and Learn" talk. Things like:

  - Book clubs
  - Collaborative katas
  - "Hackathons" (but not really, just generally working on code together)

What were some effective professional development activities that you've participated in? What were some of your favorite things about the activity? Conversely, what were some professional development activities that you've seen go poorly or that you didn't find effective?