I'd love to start a hobby project that:
* ..would teach me something new (i.e. TCP protocol, or redis, or..) * ..would keep me "entertained" for potentially 100+ hours * ..I would NOT be tempted to start monetizing. It has to be absolutely useless
As an example, some time ago I had fun implementing this book: https://www.amazon.com/Ray-Tracer-Challenge-Test-Driven-Renderer/dp/1680502719
This time I'd like to go deeper into some other topic; I don't think I spent more than 20 hours cumulatively on that book.
You could look at noncode things to learn. Make a game in a foreign language?
These days I pretty much don't start new DIY projects, I just work on existing FOSS.
Per hobby, look at various things do on frequent basis and/or infrequent basis. Try to envision how to do all/or parts as a coding hobby project. Then go do the opensource research to see if there are things that do something similar (or just specific related aspect of what want to code/do) . Jury rig / program way to get the open source things to work together.
Eample: frequent tasks/appointment & schedule reminders (enter task & do automated sms notification reminders) which would entail setting up OS, datbase (command line and/or gui), 'shell scripting' link between user / database and SMS. Perhaps upcoming scheduled report summary & automated clean-up of expired tasks/schedules.
perhaps at some point move it over to cloud so can visually access / add additional information through internet portal. might want to make sure understand security implications first.
You can learn/practice how to create the site, write about whatever you want and make it as useful or useless as you feel like.
You stated that you want a useful one in the title
Take a MOOC - this is good to explore potential topics
Ask your mom what she needs
Join a tech club