I don't know if I'd recommend Valley of Fire. That is a pretty long drive, over a hour I think, from Las Vegas.

Hoover Dam is a must see, even though they have way cut back the tour since 9/11.

The one no one else has mentioned it Springs Preserve. The outdoor exhibits are horrible, but the indoor exhibits are really interesting, and for now they have the shipwreck exhibit from Odyssey, so if you want to know what gold, silver, china, etc looks like after 100 years at the bottom of the ocean, there you go.