C.D.
Hi, K.! I just got back from a long weekend there. You didn't say when you were going, but it can be hard to do outdoor things there with the heat.
One off-the-beaten path thing we did was go to the Lied Children's Discovery Museum. It's located in a library on the very north end of Las Vegas Blvd. It's a really great, interactive museum.
We also went to the Mirage to see the Dolphin habitat and the white lions and tigers. This was very interesting, but was mostly outdoors and was very hot. Nevertheless, we got to see a newborn dolphin (only 3 hours old) swimming with his family, which was way cool.
We also caught a circus show at Circus Circus, which was hokey, but my 4 year old loved it!
Most of the high-end hotels do have kid-friendly activities, but I just listed the things we did!