Not much to add other than to reinforce what a couple others have said. I've not spent much time in Sedona, but it is worth the stop. Very beautiful area, though it does have that "small town touristy" feel if that bothers you. But it's probably the best stop-off between PHX and the Grand Canyon if you're looking for a stop.
Montezuma Castle is a nice stop as well, and literally a few minutes off the highway. It's a very small area, so we usually think of it more like a quick stop to walk around, use the restroom (though it's only a couple hours after you left!) and stretch your legs. It's very quiet and I've never seen more than a handful of other people there. Note that you can't get anywhere near the ruins, they're up on a cliff that you observe from below. But a very nice area to get some fresh air and learn about an interesting little corner of North American history, I'd I'd recommend at least stopping to check it out. For a closer look at Native American ruins, consider Wupatki National Monument north of Flagstaff.
Flagstaff has the Lowell Observatory, if you have an interest in astronomy. There's also an impressive meteor crater near Winslow, but it's a little out of the way and, in my opinion, over-priced.
For the full-on Las Vegas experience you'll want to stay on the Strip, and in my opinion the locations/hotels there are interchangeable. Save a couple hours to go downtown to Fremont Street. It's a small area and doesn't take long to see. It has the advantage of generally better gambling conditions if that's a factor, but the Fremont Street Experience is fun to see even if you don't gamble. Go at night to see the light show, which runs every hour on the hour (I think!).
