Here’s something you might have missed. The Nintendo 3DS, like the Pokéwalker that comes with Pokémon HeartGold/SoulSilver, uses the built-in gyroscope and motion-sensor to allow it to track your footsteps.
The technology isn’t as accurate as an actual pedometer, but it gets the job done in the case of the Pokéwalker, and should work at least just as well in the case of the 3DS. Once you’ve walked around a bit, you can then use a feature called “Activity Log” to count the number of steps you’ve taken, along with a graph.
The Activity Log also keeps track of the number of hours you played individual games for, as well as your total playtime in a particular day.