Advent of Code
Every december, Advent of Code hosts 49 programming challenges, which can be solved in any language you like. These problems range anywhere from as simple as adding up numbers in a list, to solving maze problems with pathfinding, or disassembling an algorithm.
I started in 2017, as an exercise in learning Rust, however, have since gone back and completed all the events, still using Rust. The source code for 2015, 2016, 2018, and 2019 can be found on my GitHub.