You were given these clues to the identity of National Park Mystery Spot 18:
Like a little less than 20 snowflakes.
Like Clint in a Spaghetti Western.
Like Superman too, in a way.
Wary and weary, they walk and watch.
An additional hint is in the puzzle's abstract, which reads:
Within this national park is a large plane figure connecting three points, one of which is wet.
The mystery spot is the Korean War Veterans Memorial, an NPS unit occupying a 2.2-acre site just southeast of the Lincoln Memorial and directly across the Reflecting Pool from the Vietnam Veterans Memorial. The memorial, which is in the shape of a triangle with a shallow pool at its apex, features 19 free-standing stainless steel statues representing a squad on patrol in rough terrain during foul weather. The Frank Gaylord-sculpted statues, no two of which are alike, depict 14 GI's, three Marines, a Navy corpsman, and an Air Force forward air observer. The poncho-clad men, who bear weapons and full combat gear, appear war-weary but vigilant.
Since the focal area of the memorial is in the shape of a large triangle with the Pool of Remembrance at its apex (see accompanying photo), it can be described as a large plane figure connecting three points, one of which is wet.
The memorial's statues are like snowflakes, in that no two are alike. There are 19 statues (that is, "a little less than 20").
In the superhero parlance, Superman is the Man of Steel.
Clint Eastwood wore a trademark poncho (aka serape) in his Spaghetti Western ("Man With No Name") trilogy (A Fistful of Dollars, For a Few Dollars More, and The Good, the Bad and the Ugly). You might also say that this squad on patrol is like the men Clint played in his Spaghetti Westerns -- silent, draped, and deadly.
In this armed men-on-patrol context, wary and weary, walk and watch needs no further explanation.
Traveler extends a Veterans Day salute to all of our military veterans. Thank you for your service to our country.