I have yet to take my son to Carlsbad Caverns, which are about an hour from my hometown.  I have taken other kids, and gone several times as a kid myself.  In fact, whenever I go I turn into a kid again.  I can't wait until my son is old enough for the trip!

Caves are cool.  In this case, I mean that both literally and figuratively.  The best time to visit Carlsbad Caverns is in the summer, so you can also watch the bat storm leave the cave at dusk, but summer can be upwards of 100 degrees here.  Make sure your hotel has a pool, and plan to spend the main part of the day underground.

Driving to the area is a miniature geology lesson - from your vantage as you descend into the Carlsbad basin, you can clearly see that this whole area was once ocean floor.  If your kids have read Edgar Rice Burroughs' John Carter books, they will immediately think of New Mexico as the dead sea bottom of Barsoom.  You can tell them that people find seashells at the tops of mountains here.

There are two basic routes for exploring the caves.  You can take the elevator down to the bottom cave (called the Big Room, and boy is it) and then walk back up (or return via elevator for little legs or freaked out kids), or do the reverse.  If you walk the caverns down to the Big Room, you can rent a headset tour, which is actually pretty good.

A cafeteria and restrooms and gift shop are on the bottom level, so you can get the lowest cheeseburger you've ever had, and the kids can get plastic bats.

At sunset, get tickets for the amphitheater seating to watch hundreds of thousands of bats spiral out of the cave mouth and into the purple sky.  From a distance they look like smoke, but up close you can see individual bats.  Last time I was there a bat got caught in a lady's hairdo, which was the highlight of the trip for the 10-year-olds I was with.

Carlsbad Caverns were my first caves, and I still adore them.