Punkva Caves are one of the classic visits in the Moravian Karst, north of Brno. Guided tours pass through large stalactite chambers, descend to the bottom of Macocha Gorge and include a boat ride on the underground Punkva River.
The caves are popular, especially in summer, so booking ahead is sensible. The trip is worth the effort if you want to see the karst landscape from the inside rather than only from the trails above.