Carlsbad Caverns
Carlsbad Caverns are typical limestone caves with typical limestone cave rock formations. They just happen to be very large. They are also well-lit (for caves), and have a paved trail running through them, which is a big deal when you consider the rough terrain inside a cave. Portions of the caves are even accessible to wheelchair users.
Another great feature of Carlsbad Caverns is the fact that the National Park Service has thoughtfully provided railings along most of the (paved) trail. That railing is the only thing that made it possible for me to walk the 2 mile trail without smashing myself to bits by tottering into walls and rock features or falling over 25 times.
I am proud of the photos I managed to take inside the caverns, not because they are in any way unique (like most of my photos, they are the same stuff everyone else takes pictures of) but because I took them without a tripod by leaning on walls and railings and holding my breath while I snapped.