The Kaumana Caves in Hawaii are actually lava tubes. Although the Thurston Lava tube is the most famous (and developed to be a pretty good tourist trap), the Kaumana Caves are undeveloped and completely natural inside. Requiring flashlights to get anywhere. I hiked it myself, but some guided tours are available.

Anyway, this is a good photo of some of the past lava movement that went through the cave. The floor is all dried lava. Mostly broken stone, but some smooth parts like this. All very interesting textures and colors. Fun hike/spelunk.