Matched Betting (not available in the USA) enables you to gamble (nearly) risk-free. I say nearly as there is always room for error! It involves placing bets on all outcomes of an event, using free online bets given out by the Bookies. Although it may take a while to get your head around the concept, Matched Betting can make a substantial addition to your monthly income.
If you're serious about making money online, start a blog. Blogging is one of the easiest and most sustainable income sources. As long as the blog is setup the right way, in the right niche, with the right content targeted at the right audience, and the offer is complementary to the content, you could make a tremendous amount of passive income from a blog.
If you have built up an email list, you could also promote your affiliate offers via email promotions. Just make sure you build up a relationship with your audience first instead of going for the hard sell straightaway. The emails you send out must contain your affiliate links to products so when your audience click through. the sale is attributed to you.
If you have a good idea for an app, and the skills to create it, then producing your own premium app can be profitable. However, with so many free apps available, it may be hard to convince people to pay. If you want to go down the premium app route, consider having a basic free version to encourage people to download the app in the first place. You can then provide the option of upgrading to a premium plan once users realize how useful the app is.