So the watch can tell you your speed and how far you've ridden based on GPS, or you can get a wheel sensor which is more precise. Personally I find GPS is enough. But speed sensors are usually bundled with cadence sensors and you probably want one of those. The Garmin or Wahoo one will work, as long as it's ANT+ any speed and cadence sensor will work with it.

There's also a quick release kit available that lets you put your watch on your handlebars or stem to see it better while you ride.