Scuba divers, lets divers log visibility and current conditions at dive sites. The idea came from common questions about conditions that people posted in Facebook groups.
It was something new that I wanted to learn how to use, and it solved the main issue of how do I easily make an app for iOS/Android.
Start with a very small goal and objective, get the idea working, don't worry too much at the start about how it looks etc.
Make it functional, if people use it, you can make it pretty later :)