We came here in early August, and played for a good 1-2 hours. Free parking in the lot next door. Don't bother paying at the kiosk outside. Entry fee is pretty hefty since kids over 1 cost the same as an adult. Eek!!
One of the funnest parts was the tower right inside the museum, that goes up and up and up. There are tons of activities inside along the way, and learning exercises. There's a water park area too on the first floor. Try the hurricane zone; it's pretty crazy and fun.
Great way to spend some time indoors on a hot summer desert day, away from the busy-ness of the Vegas Strip.