Crystal Lake Cave is a small but worthwhile underground detour just south of Dubuque. Guided tours lead through lit limestone passages lined with stalactites, stalagmites, flowstone, and other mineral formations, making it a good change of pace from the riverfront and historic districts in town.
The cave was discovered in the 19th century and later opened for visitors with walkways and lighting. Bring a layer even on warm days, and check seasonal hours before driving out; this is a simple, old-fashioned cave tour rather than a full-day attraction.