My wife and I did Red Rock. It was fun, easy. If you want a longer ride, park prior to getting to the park and use that road as a part of your ride. It is quite rideable, with good choulders. Hard to describe, as I have forgotten the names of the streets, but it should make sense when you get there.
There is also an ecological park out east of Las Vegas (but still in the town) where there is some nice, but short, riding.
Las Vegas has wide streets that are at 90 degree angles and go on forever, it seems.