Tham Chang Cave

A labyrinth with many stairs runs through the cave.
The cave is definitely worth seeing. You can also get culinary delights along the river to the cave.

Absolutely worth seeing. The hike, the ferry, the cave hike (you don't need a flashlight) and then a bath in the mountain spring

Tham Chang Cave is the only major tourist attraction within easy walking distance of Vang Vieng town.
Tham Chang Cave is open Monday to Friday from 8:00 to 11:00 a.m. and from 1:00 to 4:00 p.m., and Saturday and Sunday from 8:00 a.m. to 4:00 p.m. Entrance to the area around Tham Chang Cave costs 5,000 Lao Kip, and entry to the cave itself costs an additional 15,000 Lao Kip.
Tham Chang Cave features several stalactites and stalagmites, as well as interesting colors on the walls caused by the minerals in the water trickling through the cave.
Source and further information: laoslife.info/tham-chang-cave-vang-vieng