Learning to use Scheme better by doing Advent of Code, on absolutely no schedule at all.
Where at all possible, I use only standard Scheme functions from R⁵RS. (Even better if I can run it using my own implementation!) Otherwise, I'm using DrRacket, so I can fall back to their library from PLT Pretty Big language or (if necessary) Racket. At the very least, I'm trying to use absolutely no Racket-specific syntax.