Take a drive to the "largest and most popular caverns in Eastern America" near Virginia's Skyline Drive and Shenandoah National Park. 

Luray Caverns, a U.S. Natural Landmark, is noted for the profuse variety of formations and unsurpassed natural color.  From well-lighted, paved walkways explore cathedral-sized rooms with ceilings 10 stories high, filled with towering stone columns and crystal-clear pools.  Also, hear the hauntine sounds of the Great Stalacpipe Organ, the world's largest musical instrument.  This most unique instrument uses stalactites covering 3 1/2 acres of the surrounding caverns to produce tones of symphonic quality.

TripAdvisor Review 

While you're there, why not stay overnight in the Mimslyn Inn and travel 30 minutes the next day to Front Royal, VA to see Skyline Caverns and eat at Spelunker's Burgers and Custard?