The Open University offers short, self-study courses for free. There are a wide variety of subjects including the Arts, Business, Science, Technology and much more.
You can take a course via the website, or download a DOC/PDF/Kindle/ePub format of the course text (any interactive elements or videos will not be available in the electronic documents).
The course listings let you sort the courses by duration. The courses are very amenable to short, bite-size chunks of learning. You can complete one course section at a time rather than finish a course in one go.
https://www.open.edu/openlearn/free-courses/full-catalogue
2. BBC Bitesize
Bitesize is the BBC's support resource for school-age pupils in the UK with simple-to-follow lessons and videos. It's actually an excellent learning resource for adults too!
The explanations are a model of clear writing without dumbing-down a topic. Any grown-up reading the site can learn a lot about a topic (as well as how to write with clarity and without waffle).
Not convinced? Try this random topic and see if you like the explanation (or not!)
Fundamentals of algorithms: https://www.bbc.co.uk/bitesize/guides/zjddqhv/revision/1
You have not specified what your interests are.
There are some newsletters that deliver great content right to your inbox.
Mental Models: https://models.substack.com/
Marketing Examples: https://marketingexamples.com
Marketing Tips based on Experiments: https://tips.ariyh.com
Productivity Game: https://www.youtube.com/channel/UC02x9yG9ZFF_VZp1VnMoptg
While we are at it, I think one of the important aspects of learning is periodic recall to ensure you do not forget what you learn.
Especially this mode of learning tiny bits when you have time implies:
1. You are not part of a rigorous system like school/college that ensures learning has happened.
2. Your learning is not periodically assessed through tests and applied through assignments.
3. It takes lots of effort and discipline to ensure what you learn next builds up on what you learnt previously and not a bunch of random stuff that do not amount to anything much at the end of a year.
So definitely this is a very arduous undertaking.
With all these in mind, Udacity and Coursera offers mini-degrees which are a set of 4 to 5 related courses on a subject. Coursera atleast allows you audit those courses for free.
If anyone has a better suggestion, I would be very interested to know.
I like to pop into the MasterClass app and listen in on a class on advertising or music production. Things that I will never be good at, but often come out with some different way to view the world.
Philosophy is where spirit, drive, and purpose comes from. It's what helps you deal with stress and anxiety. Philosophy is a bit too heavy to deal with in the day, but nights are great.
If you want something both light-hearted and heavy, try https://existentialcomics.com/