Advent of Code is an Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.
Follow to adventofcode.com for details.
The challenge for me is to come algorithm first using the standard library only. The intention is to learn and feel the language in itself. I mean, there is a reason for the language to have exactly what is has in SDK.
Having all that in mind, my concern is to create the properly maintainable code, which is nice to read, understand and hopefully learn from it.