Build plugins or themes for WordPress. There are 58 million active WP sites now. There are mature markets for listing and promoting what you create. Bonus money if you can tie what you build into generating revenue for customers -- affiliate marketing tools/integrations, plugins or themes that play with existing e-commerce plugins or themes people use, marketing tools, etc. If you can do that, you can justify a $50-100 price tag, and just one sale a day will create the revenue you need.

8 years ago or so, I ran a little ecommerce site with a wholesale/drop-ship product provided by another company. When I needed extra spending money (being a full-time college student at a private university and no savings), I'd clone my own site. I'd design a new theme for the shopping cart software and sell the same product. I'd advertise it on Google AdWords for a few days to get some orders. Then I'd package that all up and list it on eBay -- a fully functional site with existing customers, a supplier, a known conversion rate, and pre-written ad copy and keyword list for the buyer. These days people use Flippa instead of eBay to sell websites, but it's something you could try as a web dev.