You can earn above average rates of return because peer-to-peer lending eliminates the bank function. That means that you participate in nearly the entire interest rate being paid by the borrower, rather than the less than 1% that you will typically earn on certificates of deposit. And you can reduce your risk by investing in slices of hundreds of different loans.
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.