The Garmin GSC10 is the accessory that you need to get your FR60, 310XT, Edge 500, and Edge 705 up to speed (if your unit didn't come bundled with them). It's a simple device that mounts on your left chainstay, and comes with both a cadence sensor magnet and speed sensor magnet. Zip it on, spin the wheels and cranks with your Garmin CPU on and let the recording begin.
The GSC10 also works with Garmin Colorado and Oregon series units, but only the cadence sensor signal gets picked up. The unit should work with other brands' ANT+ speed and cadence receivers. Claimed weight of the Garmin 2GSC10 is 25g.