Metrics measurement and tracking of some sort, depending on your area of interest and existing expertise. That could be Graphite and statsd, or a particular typ

Metrics measurement and tracking of some sort, depending on your area of interest and existing expertise. That could be Graphite and statsd, or a particular type of web analytics like Google Analytics or MixPanel. It takes roughly 40 hours to get a good basic understanding, implement, test, and start tracking useful things with each of those.