Sensor Watch. Open source replacement movement PCB for the Casio F‑91W and other similar watches. Open source firmware. ARM Cortex M0+ microcontroller that you can develop for using standard GNU toolchains. Battery lasts years. Temperature sensor makes it a temperature compensated quartz watch. They're even making custom LCDs now.

https://www.sensorwatch.net/

Wear mine every day. Contributed some improvements to the pulsometer and TOTP faces. Was even maintainer for a while. Really nice community. If you want something awesome to hack on, this is it.