In France, I can think of several regions which would meet your criteria. June or September would generally be the best months from a weather, traffic, and crowds standpoint.

The Loire valley near, but not in, Tours, would work well. There is easy terrain, a good network of minor roads, and many sights to see.

Burgundy, based near the town of Chalon-sur-Saone. There are several bike paths which crisscross the area. Away from the trails, there are more moderate roads in the pretty countryside. https://www.burgundy-by-bike.com/

The Dordogne valley. I'd suggest somewhere such as Lalinde in the west or Bretenoux in the eastern part of the valley. They would be excellent. Flats roads in the valley, moderate terrain elsewhere, and a plethora of places to visit.

In Provence, a base between Avignon & Arles would put you in an area of easy terrain with pleasant rides in various directions. May and late September would be good times here.

In the U.S., for 3 or 4 days of riding, I can think of Martha's Vineyard/Nantucket islands, the Finger Lakes, & the Pennsylvania Dutch area of Lancaster County.