Do you have a website (maybe getting traffic from SEO), a SaaS (you sell to a few people) or anything that's consistently making you money that you haven't touched for a long time?
I have a small console-based software that's generating a list of expired domains. Selling this to a guy I used to work with in the space for a low, but consistent monthly $. Doing some fixing/enhancements once a year and it's still working perfectly.