We always take guests to the underground caves. The tour is an hour and there are a ton of varieties of rock, stalactite, stalagmite, ribbon and formations to explore. Care a high powered flashlight with you so you can see more. They also offer more prolonged hikes/crawls in various holes (like 3 hours). Too confinded for me though. Kids and buy trinkets and other stuff at the top of the short train ride underground. Good tourist thing to do. Call ahead as they sometimes have school buses there and it can be a longer wait.