GRouteLoader is an IQ app/widget lets you download routes/courses directly from RWGPS or Garmin Connect to your Garmin Connect IQ enabled device using the number of the route/course.
There is the RWGPS IQ app but GRouteLoader lets you download any public route (that you have the number for).
It also lets you download locations from sendpoints.us (a website that creates a 4 digit number for a location you searched for).
There is a “widget” version (which can be used during an activity) as well as an “app” version.
Lots of detail here:
GRouteLoader can also be used to display a list of rides from websites like those run by bicycle clubs (the club has to code the website but it's not that hard).