One of the best things that I did was spending a week making a simple app that can put all my Apple Watch walks on a single big map, then sharing it with my friends after it got published on AppStore. It's been a year since I worked on it, but I still get messages from my friends (and some random people who found it!) how they've walked through an entire city or something. Really rewarding experience, despite having zero financial gains from it.
OP is right, making simple apps for your friends for fun!
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