For me it's all about learning. I typically pair something technical I want to learn and something more "creative" or outside of my primary skills, this way the learn success is guaranteed.

For learning to happen, I really need to build things in full, be cause typically you learn the details that really matter.

For example, I wanted to learn Android Things (Android in general), and I set myself to build a LED display. It turns out you can't control it via GPIO because on AT that's too slow, and you need an external "video card". Next, I was learning how to program an STM32.

My latest project is an open source security key. Eventually I want security keys to "embedded" in other objects, such as a piece of jewelry (again, mixing something I know and I want to get deeper at, hardware security, and something totally unknown, jewelry). This is a multi-step project, and the first phase is to release the security key. I just set up the website yesterday night, feedback welcome: https://solokeys.com