We were looking for something non-Strip to do while in Vegas recently, and we checked out the Mob Museum. It's in a former post-office/courthouse building, and it's a very interesting museum tour. There are three floors to explore, but you have to go through them in order. Lots of history of organized crime in general across the country. Later in the tour is Las Vegas history, which is pretty interesting. There are several short movie clips to watch at various points in the tour, and a lot of memorabilia on display.
When there are several people in the same area of the tour, there is not nearly enough seating or room to stand to watch the video clips. There is a very interesting clip about the St. Valentine's Day Massacre in one part of the museum where there is only a single small bench, and when we went there were at least a dozen people all in the same area trying to watch. The exception to this is a rather large courtroom setting for a scheduled movie session that had ample room.
We enjoyed it and it was something interesting and different to do during the day that was off the Strip. I think the admission cost is a little steep, but everything is in Vegas. It's less than 2 blocks from the Fremont Street Experience, so a very short walk gets you into the older downtown casino area around Fremont.