not trying to be cute here, but Wind Cave and Jewel Cave National Monument are (to us at least) sorta boring and are mostly just . . . well . . holes in the ground.
Seriously, there is not a huge amount of visitors at either.
Jewel Cave has over 200 mapped miles.
Wind Cave:
We were more impressed with the thousands and thousands of prairie dog holes (some even in the gravel roads) and the other wildlife than the cave(s)
Spend some time in Custer SP since you will driving very near or through there to get to Wind Cave. Much better scenery