If you were in line at the bank or something, would you play a quick, simple game on your watch? In the past, there have been several watches with built-in simple games, with modern technology the games could look a lot better, be more complex. And it shouldn't be too bad for battery life, Apple's 18 hour battery estimate includes 45 minutes a day of app usage (and the way Apple Watch currently works as just an extension to the iPhone, most of the game would be running off the phone's battery, not the watch's battery).