If you want to invest your next learning session with a code kata (TL;DR: you should), here's an opinionated list of kata catalogs:
- https://www.codurance.com/katas, code katas on different topics and with different difficulty
- https://github.com/emilybache/Racing-Car-Katas, to test your knowledge of the SOLID principles with on-hands practice
- https://matteo.vaccari.name/blog/archives/293 is also a code kata that challenge your ability to follow the OCP principle
- https://github.com/gamontal/awesome-katas, a more generic, community-driven list of katas
- https://nealford.com/katas/ and https://fundamentalsofsoftwarearchitecture.com/katas/list.html, because who said that katas are just about improving coding skills? Architectural katas are a good way to practice architectural skills!
Remember, when tackling a kata, you should first have a clear idea of what you want to learn or what skills you want to sharpen (e.g., improving TDD skills, refactoring skills, code design skills, etc.). If you want to improve a skill, try bending it to force yourself to walk on uncomfortable territories, like the Object Calisthenics does.