The activity log measures everything, not just playtime. You could leave it on the menu screen and it would tick over on the activity log.
Regardless, the fact remains that it's not a very high content game, with a whole bunch of reused assets which I've played through thoroughly before (as much as I enjoyed free flight, it's exactly the same gameplay as seen in Wii Sports Resort) for a pretty high RRP. Gyrocopter, parachuting, a new island, these kind of things could have really added a lot of value.