In the spirit of the advent of code challenges this month, I thought I'd let y'all know about a similar series of exercises: TryHackMe is a platform to learn about offensive and defensive security things.
This month is their second "Advent of Cyber," in which each day they teach you about a new security topic and provide you some free challenges to try them out yourself - they provide a video walkthrough every day as well because the intent is for you to learn. 25 days in total following the month of December.
Part of my job (unrelated to TryHackMe) involves putting together good application security training resources for our developers, and this year's Advent of Cyber is looking like a really excellent introduction to a lot of cyber topics. On Day 2, you execute a reverse shell. On Day 5, you execute SQL injection. The later days in the schedule look like we'll be hitting packet inspection and reverse engineering.
Check it out!