Generally, the buffet quality isn't that good. I've eaten at 2-3, but none have been that impressive that I'd recommend it. Be aware there are two different locations of the Grand Canyon, one is the South Rim, which is what most think of when they think of the Grand Canyon, it's about a 4 hour drive away. There's also a West Rim that's about 2 hours away. We drove all the way to the South Rim, but it was so dense in fog, you truly couldn't see anything, which made for a long day of driving with little pay off.
I've heard great things about Zion, Valley of Fire, and Red Rock Canyon, but we haven't been able to make it to any of those yet. With a week, you can probably see a few of those without a problem.
For shows, we've seen O, Ka, and Blue Man Group, of which O was probably the most impressive.