A Garmin watch. Having your health data easily available makes healthy decisions (take the stairs, or walk a bit more, or hit the gym...) a lot easier.
I see this quite often with people using smartwatches. "Oh I've hit my calorie burn target for the week, I can skip workout today". It's not just about calories but also about toning and stamina and establishing daily routine.
Having hard data in my wrist and setting up reminders and gamification helps me to remain on track.
I have daily goals that reset at midnight, so when I walk twice as much yesterday, I still have to walk my daily quota today.
It was very effective for me.
Now I'm wearing the Garmin Forerunner 935: It is more feature complete but a bit less discrete and it shows that you're wearing a sports watch.