The Blue Ridge Parkway in the mid-Atantic has massive stretches of cellular dead zones. The entire road is a popular destination, and many at the most popular destinations for views and hikes are dead zones.

Peaks of Otter comes to mind as a fantastic day-trip or weekend destination from DC. Despite being a very popular spot for day hikes, and having a lodge, it does not have cellular service. Nobody would think their rental car would be inaccessible at a popular resort area.