Strava Global Heat Map is cool. Especially for figuring out whether or not roads are passable. Caltopo is also cool, just for general map nerdery.
I figured out how to add the Strava Global Heatmap to Caltopo as layer tonight.
Open Caltopo
Click +Add New Layer
Click Add Custom Source
In the Dialog, Choose
Type: Tile
Name: Doesn't matter
URL Template: http://globalheat.strava.com/tiles/cycling/color2/{Z}/{X}/{Y}.png
Max Zoom: 18
Overlay?: Doesn't matter