This definitely resonates with me. One of my favorite pasttimes is to code away on a 2D multiplayer RPG engine, and I've ended up doing at least a few of the projects on this list as part of that (ECS framework, voxel renderer, physics engine). Integration with a scripting engine like Lua is another fun one for a list like this, especially if you try to do it in an idiomatic way for your main programming language and/or leverage code generation and metaprogramming.
One thing I've found I enjoy is working on a larger-scope toy project composed of many loosely coupled systems. It works well for me because no matter what type of project I'm interested in working on, I can usually find something applicable within the larger project. Currently on my to-do list are behavior trees and procedural terrain generation, and honestly I don't know how I'm going to decide which to do first.
One thing I've found I enjoy is working on a larger-scope toy project composed of many loosely coupled systems. It works well for me because no matter what type of project I'm interested in working on, I can usually find something applicable within the larger project. Currently on my to-do list are behavior trees and procedural terrain generation, and honestly I don't know how I'm going to decide which to do first.