The Powertap Powercal is a heart rate monitor belt with clever software in it that calculates power from heartrate. It doesn’t need fitting to pedals or crank, so you can use it on multiple bikes, even on footbikes and elliptical trainers.
The USB access to Fenix GPS watches makes it really easy to view activities and tracks on a laptop: just plug in the USB cable and you’re done. But on a phone it requires something extra: a powered USB hub. Fortunately, there are very small ones around.
Update: This article is outdated. The Garmin Connect Mobile app now (February 2018) requests your consent to upload your data. It is not necessary anymore to use the options below. This page is here for reference.