Ask HN: How do you improve skills of programmers in your team?

As a lead programmer, which approaches of improving skills in your programming team had the most positive feedback? I presume some activity not related to day-to-day job that may be considered boring, especially by junior developers. Local hackathons? Doing some over-the-weekend collaborative projects? One-hour-a-day projects? Forcing to reading some tech books? Examinations? Providing local classes, with or without homework? Going to external conferences?

It's interesting to hear your experience and attitude to these approaches.