If it's sufficiently fast and has a screen, someone will eventually get an old-school game emulator running on it. Over the weekend, iOS developer Gabriel O'Flaherty-Chan made that happen for the Apple Watch with a Game Boy emulator he calls "Giovanni."
Named for the leader of Team Rocket from the original Pokémon games, Giovanni runs Game Boy and Game Boy Color games on the Apple Watch Series 2 (the Series 1 hardware is presumably capable of the same performance, though the original Apple Watch is likely too slow). O'Flaherty-Chan used the existing Gambatte emulator as Giovanni's core and then added Apple Watch-friendly controls. There's room for the select, start, and B buttons at the bottom of the screen; you tap the right side of the screen for the A button, while the left side of the screen is mapped to the control pad. Turning the Digital Crown up and down can also simulate the up and down buttons on the control pad.
In a blog post, O'Flaherty-Chan writes that porting the emulator was complicated by the fact that watchOS supports neither OpenGL ES nor Metal, the graphics APIs used in macOS and iOS.
Giovanni exists mostly as a proof of concept. Apple would never allow the app to be distributed through its App Store, both because it's doing things the Apple Watch isn't meant to do and because it doesn't allow emulators on any of its platforms. And it has a number of bugs that O'Flaherty-Chan blames on "the constraints of watchOS," including crashes, occasionally garbled graphics, and some speed hitches depending on the game. But if you want to tinker around and get it working anyway, the source code is available on Github for anyone who wants to build it and sideload it onto their watches.