I recommend Advent of Code to everyone I meet. It is a wonderful way to develop your programming skills, since you’re completely on your own (other sites babysit you quite a bit, methinks). However, while doing the implementation is a great exercise, it is much more useful (especially if learning a new language) to look at what others have done (after solving it ourselves!).
I did about half of AoC in 2019, and figured I’d work on the 2020 edition when I had the time. Well, here we go!