Advent of Code

As a self confessed geek, one of the things I do for fun in December is Advent of Code.  It's a series of puzzles, one for each day in advent, that can be solved by writing some computer code.  There's a story attached that usually involves saving Christmas.  

You can use any language, and there's usually a simple solution that completes quickly, though it can be hard to find!  It does get fiendishly difficult from a gentle start, I still haven't managed to complete the full set.

Recommended for those of you with a technical bent - some of them can even be completed using a spreadsheet.

https://adventofcode.com/ for more details.