Writing is very underrated way of thinking, but I can recommend it highly. Start journaling and/or writing essays. Just write down what you think, feel, want to communicate, and what you understand. Just be clear and honest. pg has repeatedly written about this [0]. You can also choose to write technical posts.
Solving Math problems and competitive programming problems also enhace thinking.
Teaching is an underrated way, too. When you teach a hard concept, you really need to think a lot about how to get the idea across, how to make it accessible to different kinds of pupils.
[0]: https://paulgraham.com/writing44.html