I made my bookmark uploader on the train coming home from New Adventures. The idea was simple, I wanted a way to easily upload my bookmarks section. I managed to build an app for my iPhone that let me share from Safari and uploaded to my CraftCMS site via GraphQL. This is no longer in use because I've moved to a static site to reduce server costs, but the idea in principle was very good and made the process of storing bookmarks on my site as easy as saving a bookmark in a browser.
Takeaway from project: iOS development experience. Making a very simple MVP in hours not months. First time using GraphQL, it worked really well. Reused code from previous project (AR Bookmarks).