Advent of Code
If you're not doing Advent of Code in December, you're missing out. I use it to keep sharp, and to study languages I aspire to learn. My current aspiration is to spend more time writing Rust, so there's a lot of that there.
Learn to code / Race your friends
Advent of Code is an annual daily challenge running from December 1st through Christmas. It's a great combination of engaging, impossible, humorous, and possibly not impossible to make your December even more special. I'm certainly not through all years since 2015 (I only started in 2020), but I occassionally come back to them in my spare time.