Every single NP is unique and special
Groom Lake is going to be tough unless you plan to use a parachute...
Every single National Park is unique and special -- I have been to 35 or so.
Devils Tower is cool, but a lot tougher to reach the base than was depicted in Close Encounters.
World land speed record & mysterious sliding rocks
Forget the salt flats -- go for the world speed record -- set at the Black Rock Desert in Northern Nevada. Also check out the famous Guru Drive (http://www.roadtripamerica.com/slides/guru/guru01.htm) and if you are feeling adventurous -- the lake bed is ringed with hot springs, including one spot that has three volcano-like vents and HOT waterfalls cascading into the pond.
Another famous mystery spot is the "sliding rocks" of Racetrack Playa in Death Valley (http://wrgis.wr.usgs.gov/docs/parks/deva/galrac.html)