I had an idea a little while ago: constantly rewriting code. It was inspired by the idea of doing something repeatedly allows you to improve. So constantly rewriting the same bit of code (with changes) would allow you to get better at writing that sort of code (or solving that sort of problem). Every time you'd end up doing it slightly differently and learning something.

I also came up with a different idea along the same lines while writing this comment. Weakening it to rewriting code regularly (instead of constantly) and adding a (rough) time limit. A reasonable time limit could be anywhere from a day to a week. I think it'd work like this: you'd build a project for the first time (in a few days) and use it for things. Later when you find a problem with it, you'd return and rewrite the whole thing (again in just a few days). I think the combination of rewriting from scratch and a time limit would limit the complexity of each project. One thing I haven't fleshed out yet is how to limit the depth/complexity of dependency chains.