Looking at your restrictions I would think the Delmarva area. Also know as the Eastern Shore of MD, VA and DE. I like biking there, flat, scenic and historic. Enough towns to grab a pint, little vehicle traffic, state camping here and there, ocean,rivers and bay views, pines, rural farmlands. Early May is a good time to go there.
Great sites:
Harriet Tubman museum and Freedom trail (mural!!)
Blackwater Wildlife Refuge (birders haven)
Assateague Island
Pocomoke River
Chincoteague
Prime Hook Wildlife refuge
Cape Henlopen
Trap Pond
Tuckahoe State PArk
BombayWildlife Refuge
And on and on.......
Ideally, the SW (Utah/AZ)is best but a long haul in a car.