Not only will this multiply the money you’re bringing in in a serious way, but it protects you against any sudden changes in the market or in your business. Remember that old saying about putting all your eggs in one basket? A few hours a week committed to just one or two of the following opportunities will put you in a much stronger position to be financially safe and independent.
Outbrain is not necessarily the network of choice, it was just an example. As for tracking the ROI usually you'll want to track campaigns in Google analytics and track clicks on affiliate links as event and average value/email or you'll want to duplicate the post and run paid traffic to the duplicate so you know exactly what the conversions are as you can put them in separate email lists etc too.
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.