Ask HN: What to do with 1 hour per day?

Hi, after reading some productivity blogs I realize how much I can improve myself spending 1 hour per day in an activity.

In a normal day, I usually have the following activities: programming(Python/Django), sports(tennis, running), studying.

I was thinking about the best activity in which I would invest my extra time, the activity must be something that I could catch up right away and learn/make something useful.

I want to increase my problem solving skills and I think some course like SICP could be a good idea but I don't know if I would like to consider another options.

Thank you for your time.