I have good news and bad news: you can totally get little contract jobs on the side for extra cash as a developer, but you have to put time and effort into drumming up clients, list as if you were doing contract development full time. You have to convince each new client that you're worth their money. I've had some luck doing contract work for previous employers that already know I'm trustworthy, but I can't just turn it on and off like a money faucet.
Your other option is to make something and sell it yourself, which is even more of an up-front investment of time with less guarantee of a return. You'll be spending even more time doing marketing and business management.
So, the short version is you're not likely to get easy money without putting more effort into it, unless you've already built up numerous business relationships. Sorry! I wish I could just write useful code and get paid for it too, but it turns out I have to keep convincing someone to give me money for my work.