Heart Rate Monitor - to stay in the training zone. Also, I have noticed that elevated HR helps with my occasional triggeless anxiety.

HRM is a duo of a computer watch and a strap/belt that you wear around your chest. You see your HR on the watch. Or, if you exercise at the gym as I do about 1/3 of the time, the gym machines read your heart rate. And, it is much more precise than the reading from the sensor handles of the gym machines. Plus, when you walk and especially run, you do not want to hold those sensor handles - you want to move your arms naturally. So that's why I splurged .