Lurgrotte is a major limestone cave system north of Graz, known for guided visits through chambers, dripstone formations, and underground passages. It is a good change of pace from the city's Renaissance courtyards and hilltop viewpoints.
The cave is a day-trip style attraction, so check the current entrance point, tour times, and transport before setting out. Bring a layer even in summer; underground Graz is much cooler than the streets around the Hauptplatz.