One of the natural wonders of the world. If you can spare the time, you must visit.
Words really aren't enough to describe the Grand Canyon. The sheer size, that warps your sense of perspective and makes you feel you're falling into an Escher painting. The way the clouds mottle the canyon below. The beauty of the place - it really is breathtaking. Unfortunately, it's also a long way away - the South Rim (the most beautiful part, and the one you see in all the textbooks) is 279 miles away and a five hour drive.
Yes, you can do it in a daytrip if you set out at crack of dawn and drive like the clappers - it's more than worth it, even for an hour there. And yes, you can stay the night in pretty Williams, AZ - the nearest town to the canyon. But if you don't have time, I really recommend getting a helicopter or airplane tour, which will whisk you there and back from Vegas in a matter of hours. It's expensive - a minimum of US$250 upwards per person - but you won't regret it. There are countless companies offering the tours, too, so you can try and beat them down on prices (usually they'll waive the entrance fees for the national park or throw in a free lunch, but every little helps). I have friends who've used Papillon Airways (www.papillon.com) and reported back good things.