Great suggestions here so far and a common thread I noticed is leveraging platforms such as Slack/Gmail/Trello/Shopify etc. - essentially an B2B platforms. My friend and I currently run cardsync.xyz on Trello which allows Trello users to sync their cards and it generates under $1000/month. We spend about 2-10 hours a week on it.

You will be at the mercy of these platforms when (not if) they decide to change their rules but until then, it can be a good source of side income.