A few years ago, just for fun, I enrolled in a UCLA extension course on Measure Theory: https://uclaextension.edu/pages/Course.aspx?reg=Z5529
It stretched my brain in ways that my day job doesn't. I've since continued to take all of the courses in the series, which are always about "interesting" mathematics. The professor (Dr. Michael Miller) has been teaching these graduate-level extension courses for something like 20+ years. If you are in the area of UCLA, I highly recommend attending. Sadly, there's no online option, so you'll have to be a local.
More info on the course from another student (and clear fan): http://boffosocko.com/2015/09/22/dr-michael-miller-math-clas...