Walking. Whether it be inside or out, with music or without, intensive or relaxing, it's a simple thing that's good for your health and largely commitment-free.
Like Rocinate said, watching youtube and anime, whatever suits you best
Drawing, writing, painting, sculpting, other fine-art type things that require large amounts of concentration and practice.
If you have the means, cooking or especially baking is a very rewarding hobby. Plus, you can gift your treats to others / indulge yourself, good for mental health
Learn a language - duolingo is indeed free
Hope this helps bro