Fitbit flex

Just got mine yesterday, but I had an older one. The Fitbit will only log steps and intensity, in other words, it will be able to tell when you are running, walking with a purpose, or walking leisurely. Check out your fitbit dashboard after your first day to get a better idea of what it looks like. The fitbits also log the time that they are measuring the steps, so if you add football from 1200 - 1300, MFP will adjust the amount of adjustment that comes from the Fitbit from 1200 - 1300. When I used MFP with the older Fitbit, I would still log the activity and didn't have any problems with calorie deficit (I'm guessing you are fearful that by using the Fitbit and logging an activity, it will double deduct and therefore not give you a deficit - it doesn't).

Hope this helps!

Oh, and PS, since the Fitbit is just logging steps/intensity, you will get more calories for specific activities that might not be reflected through steps/intensity.