I haven't seen "love" but have been told its the best of all the Cirque shows out there by someone that has seen most of them. If you've seen many Cirque shows, then don't bother w/ Ka' (the one at MGM). O was pretty good, but after seeing a lot of the stunts at other Cirque shows, Ka' losses some of the typical Cirque 'magic'. yeah there is a huge moving/vertical wall climbing thing that is cool, but there isn't the multiple stunts w/ people flying 50' off the ground that knocks you off your feet.
My Wife went out to the Grand Canyon on some sort of travel package when she was out there 5 years ago, I think they had to leave pretty early and didn't get home until late afternoon. She did enjoy it, but unless your willing to give up a whole day it may not be for you. However, since you've been to Vegas several times, it could make for a nice diversion from the typical.
I've only seen the Grand Canyon from the window of a plane, but it was definently awe inspiring. I don't think I've ever heard somebody say otherwise, one of those things you've got to see to believe if you will.
As far as Hoover damn, It probably depends on what interests you. To some its a huge engineering marvel with a great history, to others its a large concrete wall.