My husband and I will have 6 days there and wonder about some easy bike routes for me and more challenging for him. Perfer one central location/base.
There are books and sources online, but your very best resource will probably be the person from whom you rent your bicycles. Nothing beats local knowledge. Bring a good map and let him or her mark it up for you.
The best bikes you are likely to find are VTTs--what we would call a hybrid, with upright handlebars, thick tires, and many gears. Road bikes are hard to rent in France. The VTTs do very well on dirt paths, hills, and cobblestones.
For a base, you probably want to be someplace in the Rhone valley or flat lands around it, not up in the hills. That way your husband can choose more-challenging terrain but you won't have to. I would think Arles or San Remy would make a fine base, but there are many other possiblities. For a six-day stay you might find an agreeable rental (I mean, a place to stay) almost anywhere.