The venerable http://www.vttrack.fr/ is your friend.
Start by typing ‘Nice’ into the search bar to get to the right area.
You can then select the type of map or aerial photo you want to see.
IGN is the equivalent of the OS and their maps are great. At the highest IGN zoom level with contours, tracks are usually:
Two solid lines (yellow between) = minor road
Two solid lines (white between) = unmade road, regularly maintained
One solid / one dotted = unmade road, not regularly maintained
One solid line = doubletrack
One dotted line = singletrack
Note that Google appear to have licensed the IGN track database, so most if not all of the tracks will also appear on Google Maps.
You can then choose which websites’ routes to overlay onto the main map, and click on the routes to take you to the route details, often including GPS/GPX download.
It’s all made a lot easier if you use Google Chrome’s auto-translation feature, unless you speak French of course…