You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
But don't make the mistake of thinking this will be a passive source of income—you're on call whenever you have a guest and you'll always need to keep the place clean for incoming visitors. On top of just renting on Airbnb, consider offering your guests paid add-ons, like Lauren Gheysens', Royal Day Out in London, England—where she gives visitors a local's only tour of the city, complete with bespoke 18th century costumes.
Find a profitable niche: We’ve talked about this a lot. But, where are you most comfortable. What niche do your skills, values, and interests intersect? Do you have 10 years of experience as a technical writer? Do you have long-standing PR relationships that’ll be invaluable in helping startups launch a successful crowdfunding campaign? Determine what makes your value unique, and lean heavily on showcasing that strength to your potential clients.
Take surveys. You can earn $50 to $100 per month in cash and products by taking online surveys. Find survey sites by searching for “paid survey sites” online. Sign up for several survey sites to increase your chances of being selected for higher-paying surveys. Register with an email address, and check your email often so you can respond quickly to survey offers.
There is also Facebook, Instagram, Pinterest, LinkedIn, and Twitter training for those that want to get social reach. There is paid traffic training, using PPC channels like Facebook Ads, BingAds, and Google Ads. And you can even get help creating and operating VIRAL campaigns which can happen if you create your content in a specific way. This can be a powerful approach as well.