For the last few years I’ve been doing this fun advent calendar called Advent of Code - every day you get two puzzles to solve with code (some people manage it on paper or with Excel too which is amazing!)
This year is continuing as tradition has shown me it will: solve day one part 1 in minutes (can be done in Shortcuts!), and then the machine gets stuck calculating part 2 
- but it is fun!
How much experience required? Can mere non-coding mortals play? 
It varies, some challenges will be quite easy whereas others require experience with algorithms - I encourage you to give it a try regardless!
From looking at just the first puzzle I agree with Rosemary’s assessment. I wonder, though, why it is necessary to enter your GitHub/Twitter/Google/Reddit credentials. I am reluctant to do so.
This is super cool - thanks! I will give it a shot.
You can do it anonymously (after login) - however they ask for credentials because the server gets slammed by people/bots making requests and in order to avoid issues they have a login wall.