UML. Technically this is for a master's, but that's my spare time. I do find it interesting to see why it's not really used. So verbose, but not detailed enough for great communication.
Not really in my spare time since I took a couple months off from working (have to go pound the pavement this month,) but machine learning (UofW Coursera's specialization) & pandas. Also, spending time in the mornings on math (currently probability.)