If you are building a site that has the potential for information that will never age and remain useful for your audience, you have the opportunity to create what is known as evergreen content. It's important to carry out extensive keyword research before planning any evergreen content for a site like this, as your site could hugely benefit from the proper usage of keywords within such content.
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.