My 13-year old son and I have been working on a behavior-oriented personal finance tool. I've kicked this idea around for years, and we had a chance to use it as our subject for a 12-week entrepreneurial program. I built a version of it for personal use years ago, but had fun looking at it through the lens of a more serious endeavor.

It's not feature complete, but the demo of the SMS-based journaling input works pretty well: https://spendlight.app/demo