For example, for a TODO app, charge for more than a certain number of projects, recurring tasks, integration with some other service, notifications, shared task list, etc. I didn't look into the specifics of your app, but that's the general free -> paying pipeline.
But don't just randomly develop stuff. Figure out how your users use your product and add stuff they may want to a paid tier. Maybe that's asking them to take a survey, maybe it's some form of telemetry, that's up to you, but you need to get an idea of what your customers value.