One tip I've picked up from experience: avoid projects that have user accounts.
If your project has user accounts, you'll get users who you have to take responsibility for. Then it's not a side-project any more, it's a part-time job.
These days I much prefer side projects which have no user accounts at all (like https://www.niche-museums.com/) or that are open source tools where the end user installs and uses the software without me having any involvement at all (like https://github.com/simonw/shot-scraper)
If your project has user accounts, you'll get users who you have to take responsibility for. Then it's not a side-project any more, it's a part-time job.
These days I much prefer side projects which have no user accounts at all (like https://www.niche-museums.com/) or that are open source tools where the end user installs and uses the software without me having any involvement at all (like https://github.com/simonw/shot-scraper)