Carlsbad Caverns National Park was fantastic! I really didn't know what to expect... well, besides a cave. But I ended up being stunned by the sheer scale of the cave and the complexity and size of many of the formations. The Big Room at Carlsbad Caverns is about 600,000 square feet, roughly the same size as 14 football fields, and in some places its hundreds of feet tall.
We took the natural entrance down, which involves walking down a little more than a mile along a path that leads to the larger caverns. Doing this, we also got to see the swarms of bats that circle around the entrance. But if you're short on time or can't walk the distance, there are also elevators that can take you down the 700+ feet to the cavern floor. Most people who walk in from the natural entrance then take the elevators back up, but you could also do it the other way around.
Its hard to really put into words how impressive these caverns actually are, so I hope the photos I took (see below) do it justice. We only had the morning, so just did the self guided natural entrance walk (less than two miles) and then also the self guided Big Room tour. These are both free, and should be on your must-do list, with entrance to the park. You can also pay to get several different ranger-guided tours, which I suspect are fascinating.
Pro Tip: When you go (and I say "when" not "if" because you really have to check it out at least once!) arrive EARLY. We were there on a Wednesday, and it was already crowded by 11am. But we were among the first people to walk down into the hole at 8:30am, which meant we had few folks around us and lots of quiet to enjoy the formations.