We have covered plenty of cool Arduino projects here in the past. This Arduino Pedometer Watch shared by benhur.goncalves sounds like a fun project though. The developer used a sensor board with compass, pressure/altitude/temperature sensors, an accelerometer, and a gyroscope to complete the project:
The above video gives you a quick look at this watch. As the developer explains, the battery lasts 24 hours or more depending on how you use the watch. More information is available here.