I flew into Vegas in early May last year and drove a few hundred miles over the course of ten days. You might consider Snow Canyon, fifteen minutes outside St. George, UT. St. George is about 120 miles NE of Las Vegas on Rt. 15 (maybe farther than you want to drive). Anyway, it's an interesting area to hike though; lots of varied terrain; good sunrise red rock possibilities, volcanic lava fields (watch your step!) with nice clumps of wildflowers and claretcup cactus blooms, although most of the flowers may be past prime by the end of May. I've explored only a small portion of it. You can find more info in Laurent Martres' book, Photographing the Southwest Vol. 1, Utah. For that matter, buy all three volumes in the series. They contain a wealth of useful information and will whet your appetite for further visits. I highly recommend them.
Merrill