I've had several and they've all been great learning experiences. I started by customizing some php-mysql form generator and selling databases to small business

I've had several and they've all been great learning experiences. I started by customizing some php-mysql form generator and selling databases to small businesses. Next I learned ASP.NET and wrote web services to collect water treatment data remotely. After that, the biggest and most successful side project I did was Cupcake Wrapper Creator. I rewrote it once. Then I started a SaaS project in Django, but never finished. Recently I gave up on a AVR emulator I was writing (in C, to learn C at the same time). The core controller structure and instructions were fun, but when it came time to implement the IO, I realized I just wasn't that interested. Plus I have older kids now who realize when I'm neglecting them, a bigger house to maintain and a more demanding job. Now I just do a few algorithm exercises in Juypter or crypto challenges in Go each week. I'm still learning and I get a sense of accomplishment without the nagging unfinished project aftertaste.