Enjoy your free time and make yourself a better person. Here are some ways to do so.
Learn things. My last big learning project was mathematics (spent five structured years on that one). Now I'm learning Japanese. No purpose other than that I want to know more than I know now.
Read more. Read some history. Read some classics. Binge read Murakami.
Pick a couple of discrete technical aspects of some tool you never really understood, and hit them hard. I did this with C++ move semantics, and the shiny new template concepts. It's very satisfying to hit hard something you know you don't understand, and come away a few days later knowing it backwards.
Get more exercise.