We enjoyed:
The Mob Museum: https://themobmuseum.org
Valley of Fire State Park: http://parks.nv.gov/parks/valley-of-fire
Spring Mountain Ranch State Park: http://parks.nv.gov/parks/spring-mountain-ranch
*Be sure you take the tour, because as one of the first ranches in the area, it has some interesting history, and has also been owned by some famous people like radio star Checster Lauck, actress Vera Krupp, and Howard Hughes, before becoming a state park. You can combine it with a trip to Red Rock Canyon, because it's near by.
Thought of one more:
Clark County Museum: https://www.vegas.com/attractions/off-the-strip/clark-county-museum/
It has a museum (obviously) but it also has a recreation of a small US main street of the early 1900's with fully furnished and decorated buildings which are all original restored buildings. It was really charming and interesting:
The museum's Heritage Street takes you through a time warp. Not only will you forget you're in Vegas, you'll feel like you traveled back 100 years. The tree-lined streets provide ample shading on hot summer days and the manicured lawns and rabbits hopping around add a pleasant touch.
Explore five fully-furnished houses ranging from the early 1900s to the 1950s. Built in 1912, the Beckley house is a California bungalow-style house that was once located on Fourth Street in downtown Las Vegas. At the time, it cost only $2,500. The Beckley house was the last pioneer home in the area and moved to the museum in 1979.
Built in 1931, the Goumond House was glamorous for its time. The mint green walls and pastel-colored interior give you an idea of the style of the era. The bathroom even features colored toilet paper (remember those?). Also in the Goumond House, you'll see a room filled with old TVs and record players from the 1950s.
Just steps away, the Candlelight Wedding Chapel was once located on the Las Vegas Strip across from the Riviera hotel. Built in 1966, celebrities who were married in this chapel include Bette Midler, Whoopi Goldberg and Ray Liotta, to name a few. The chapel closed in 2004 and was relocated to the Clark County Museum in 2007. The chapel is fully furnished with pews, flowers and mannequin couples. You can even browse through wedding albums featuring couples who tied the knot in this chapel.
Donald W. Reynolds Print Shop is a replica of what you would have seen in the 1890s. The shop includes gigantic machines, including a national paper cutter, also known as the "guillotine cutter." This machine was capable of slicing through a four-inch thick pile of newspaper. This cutter was also used for books and magazines.
You'll also see a fully furnished trailer home, trains and historic vehicles. If it's not too hot, you can take the Mojave Desert Trail, which includes a ghost town with a blacksmith shop and a jailhouse.
*It's on the way to the Hoover Dam if you want to combine trips. We visited it after the Ethel M. Chocolate Factory, so I don't think it's too far away, or at least in the same general direction.
Ethel M Chocolate Factory: https://www.ethelm.com/category/visiting+the+factory/locations.do