I have three that cost me nothing and are extremely helpful: Walking out and about in nature -exercise, and the chance to think Listening to podcasts - I listen a a mix of political, educational and comedy podcast - often combined with a walk. Reading - I read a lot of stuff, often borrowing books from the library or free ebook repositories.
Also lots of people do some gardening, but personally it's not for me.
i try and pick hobbies that are removed from my day job or online activities