A pedometer. This is a small clip like device that measures the number of steps you take. It also measures the flights of stairs that you climb and estimates calories burned. It is synced with your computer and keeps track of calories eaten if you fill that in by computer or smart phone. You can look at how you do compared to others your age.
Fitbit. Its a special pedometer that works on your acceleration through space....Which means, you can't fool it. It enables one to monitor different biometrics like steps walking, miles, sleep, and link to other apps like my fitness pal. Some are clips, some are bracelets, some come in different colors. It depends on the amount of money you are willing to spend. I have tried and it's well worth the $.