The Museum of the Atom (or something like that) It is on the campus of UNLV near the corner of Flamingo and Paradise. It is a small museum by big city standard but it has excellent displays of the history of testing the atomic bomb in the Nevada desert. Displays, running video clips from newscasts and TV show, equipment. It was a truly interesting experience.
The Aquarium at the Mandala Bay Hotel is lovely. We enjoy the Dolphin experience at the Mirage. For a day drive head East to the Valley of Fire. Beautiful desert environment, petrographs, and if you want a full day of touring you can take the back road (fully paved highway) all the way back to Las Vegas. Boulder City has many antique stores and museums.