I have found the best side projects to be monetized versions of stuff which I built for my startup to save me time. A recent example is a script which I origin

I have found the best side projects to be monetized versions of stuff which I built for my startup to save me time. A recent example is a script which I originally hacked together to check for possible IP changes/submissions (copyrights, patents, trademarks etc) which might affect my startup's own IP. It didn't cost very much to SaaS-ify the original script (outsourced using upwork.com) and after sending out a few emails and shoutouts in newsletters, events etc I have a pretty decent side-income from private clients and law firms (pays the rent & yearly holiday hopefully). The hard part is making sure you don't let it distract you from your main project/job - I am at 3-4h/month and don't let myself go above that!