Ask HN: I have 3 months of free time, what should I do?

Hi HN,

I'm an dev in Europe with about 7 years of experience, mainly in web development (front-end and back-end) and I have a master in CS. For various reasons, I happen to have 3 months of free time (6 to 8 hours during daytime) from now until August (I can live without earning money during those 3 months).

I figured it might be good to broaden my knowledge in another IT field and start doing something else than webapps... or not(?)

I'm considering a few things to do and invest my time in during those 3 months and I can't really choose what would be best to secure a path for the next 5 years of my career:

  - take some MOOCS to try to land a job in Data Science/ML (is 3 months enough time?)
  - learn as much as I can about blockchains (is it really worth it?)
  - learn modern JAVA and its current ecosystem (my last contact with JAVA was at uni 10 years ago)
  - sit back, relax and enjoy doing nothing, for once (and only? no mortgage, no children, for now)

What do you guys think a good investment would be?