The Lanquin cave is a popular destination, and while it's worth visiting if you have some extra time on a trip to Semuc Champey, I find it a little disappointing. The geological formations are more interesting at the Grutas del Rey Marcos and the Candelaria Caves, while the adventure is much greater at Kan' Ba. Leaving little to the imagination, the locals have named many of the Lanquin stalactite and stalagmite formations after the animals they resemble.
The best time to visit Lanquin is in the late afternoon, when thousands of bats exit the cave to feed. There's good swimming in the river near the mouth of the cave, and you can even camp here. The small town of Lanquin (which I prefer over the cave) has a number of churches, including one that dates to the 1500s.