I did a PhD part time - took me about 8 years of evenings and weekends. When I handed my thesis in, it was a massive shock to the system: I just didn't know what to do with that time.
So I did a distance learning CPL! That was quite easy by comparison, and a pleasant wind-down.
Since then, I just use the time writing. Hopefully at-least one new book next year: a university textbook on airworthiness, maybe a second on a non-aviation hobby.
Yes, I am that complete and utter glutton for punishment! It does become a habit, but I think it may also be in the genes to some extent - when I was growing up my dad was always studying something - first a couple of HNCs, then a diploma in something, then a few foreign languages. My brother just keeps adding languages.