The 3DS has an inbuilt activity log like the Wii to track how long you've been playing games, but also monitors your walkabouts.
Whilst the log notes how many hours you'd have spent pounding Ryu into submission or driving around Mario Circuit, it'll also use the built-in accelerometer and gyroscope to track how many steps you've taken. Are you a fittie, or a lazy bum? The 3DS will tell you what category you'd fall into.