I usually just drink when I'm thirsty, but if you'll be walking for fitness on a hot summer day, then strap on this cool new Dehydration Preventing Pedometer. This high-tech pedometer not only tracks the elapsed time, number of steps and total distance traveled, it also automatically monitors the temperature and relative humidity to predict fluid loss and alerts you when it's time to re-hydrate. It features a large LCD screen display, tracks your activity using an accurate piezoelectric sensor, saves data from a prior workout and includes a quick-release belt clip. You really can't have too much information, but unfortunately you'll need to remember to bring water yourself.
Dehydration Preventing Pedometer