Last year, my wife and I were planning a 12 day trip and I was struggling with Google Docs moving days around while we planned our final route.
Told her to give me 2 days and ended up making a small app to help us plan our trip; the first version was built over New Years weekend:
Kept working on it and it was good enough to share to Reddit 6 days later [0]. Got a good number of users and still keep it maintained/updated. Also have some videos showing the progress and major features over the last year [1]. It's been fun; my go-to project to tinker on when I feel bored.
Tried to monetize it, but didn't really find a market so just keep it free for the users that are on the platform :) The folks that come back to it tend to be obsessive planners (like my spouse).
[0] https://www.reddit.com/r/TravelHacks/comments/105s90u/i_buil...