Although not a NP, The Craters of the Moon National Monument and Preserve is neat and a has a nice camping area.
Agree. Also on the "not so well known" list may be:
Great Sand Dunes (CO)
Russell Cave (GA)
Golden Spike (UT)
Aztec Ruins (NM)
Sunset Crater/Wupatki (AZ)
I think all of these may be national monuments as opposed to national parks, however each was interesting in their own right. I'm one of those eccentrics that looks for out-of-the-way places to visit on our trips and we've yet to be disappointed.
Funny Great Basin wasn't mentioned, it used to be the most under-visited park in the US.
Most of the GBNP visitors never get past Lehman Cave.
Last summer we hiked into the Bristlecone Grove and the next day to the Lexington Arch. The Arch was quite a hike but worth it. The road to the Arch trailhead is very rough, and not well marked. We crawled in 4wd. We saw a sage grouse doing its lek from the trail.
So many campsites, so little time...