There are two parks under the National Park Service, involving caves, right near each other in the Black Hills part of Southwest South Dakota. They are Jewel Cave and Wind Cave. There is a bit of difference between the two caves and the formations therein and both are worth visiting but if you only have time to choose one, I would recommend Wind Cave. Wind Cave is more unique and there is more to do other than the cave.
Wind Cave does not have the usual stalactites (formations coming down from the ceiling) and stalagmites (formations rising from the ground). It instead has an intricate pattern of thin lines called a "Boxwork Formation". This is because Wind Cave is a dry cave as opposed to the much more common wet caves. Wind cave contains 98% of the world's known Boxwork Formations.
I was surprised by the number of, and variety of, animals in the park. I saw no wildlife in nearby Jewel Cave National Monument except birds. Wind Cave has a decent sized herd of Americak Bison (commonly, mistakenly, referred to as a "Buffalo").
The wildlife is definitely not confined to bison, there are a number of other animal species here too. I saw a couple of small herds of pronghorn antelope.
Fondest memory: I enjoyed my tour of the cave very much. It is interesting that 95% of the world's boxwork formation is right here.
Custer State Park is just north of Wind Cave National Park. It's a fully feature park. Lodging, hiking, fishing, off-road, boating, camping, sightseeing, and wildlife.
Things to do:
Peter Norbeck Scenic Byway (Needles Highway)
Wildlife Loop Road Scenic Byway (Buffalo Corrals)
Visitor Centers (Peter Norbeck Center, or the Wildlife Station Visitor Center)
Monday Night Guest Speaker Series
Black Hills Playhouse
The Badger Hole
Gold Panning Demonstrations
Junior Naturalist Program
Fondest memory: Waking up in the morning with the wind in the pines, the sweet smell of wood smoke and a hot breakfast at the Blue Bell Inn.
Favorite thing: Since this area has many National Parks, if you plan to visit several parks in a year time, get a parks pass. It costs $50. You get free admission to most parks. It does not give you a discount on cave tours and you still have to pay the ahem. **parking fee** at Mt. Rushmore. Even if we won't spend 50 on park admissions we get it in order to support the Park Service.