I'd like to visit the Grand Canyon south rim for a day while I'm in Las Vegas. I've seen many ads for day trips via bus to the Grand Canyon, but I can't decide between them. Some cost only while others cost 0 -- and the intineries seem identical. Basically, I want a quality tour from a reputable tour operator and I want to have plenty of time at the Grand Canyon, as opposed to freqent stops at stores and attractions where the tour operators gets a commission for bring tourists there. Any advice, links to tour operators?