I had downloaded a bunch of older Advent of Code exercises and spent some time on the transits solving them in a language that was relatively new to me (Clojure).
That might be better suited to a casual/low-stakes "15-20 total hours" rather than 100 hours where you want to be sure you can be productive, but all I did was just made sure I could do a couple of hours worth of work on my laptop at home with the wifi disconnected.