Airbnb. Keep a room in your house open, if you live in a desirable neighborhood, and have people rotating through. If you charge high enough you could at least make up your rent and on good months make more. Keep a look out for big events like Outside Lands where there are hundreds of people coming through the city and up your prices.
The easiest way to make extra money is to use the assets you already have, like your house. Even if you don't have a spare room, keep a look out for those weekends and sleep on a friends couch or something. Look into your car and GetAround etc.