Oh, I can't wait to go back to the Grand Canyon! We were there July 2004, and it is absolutely magnificent. You see all the pictures and you know it's going to be great, but to actually SEE it in person - words can't describe it.
Ok, a few suggestions: there are 4 hotels on the South Rim - El Tovar, Kachina, Thunderbird, Bright Angel. The El Tovar is the fanciest and also the only one that has air conditioning (at least last year it was the only one). But you really don't need the AC, even when we were there in July. Once you figure out your travel dates, MAKE YOUR HOTEL RESERVATIONS ASAP! I made our reservations in Oct. for July, and the El Tovar was already booked. We stayed at the Kachina, which was very nice. The rooms are small, but (just like Disney) you're really not in your room very much. All 4 hotels are right on the rim - I think it was $10 or $20 extra per night at the Kachina for a canyon view room, and it was worth it.
We stayed 2 nights, mostly because we knew our 2 DS's (they were 12 and 14) couldn't handle that much "nature" without a lot of whining. Three nights would have been ideal; if you're into hiking, add another night or two.
The mule ride to the bottom: if you plan on doing this, you'll also have to make your reservations as far in advance as possible. We didn't do this: none of us had ever been on a horse before and we figured a 4-hour mule ride along those narrow trails wasn't a good way to start! Plus, there's a weight limit of 200 (or 210?) pounds, fully clothed carrying water, etc. DH weighs 200 to begin with, so he probably wouldn't have been able to go. Turns out they had a horrible rain/hail storm the day before we arrived and the trail was wiped out, so they canceled the mule rides anyway.
There are stables right before you enter the park (I can't think of the name right now), and they offer trail rides thru the Kaibab National Forest that surrounds Grand Canyon National Park, on nice, flat terrain. We took an hour-long trail ride and it was really nice. There's also a company (Grand Canyon Jeep Tours) that will take you on a jeep tour late afternoon to another section of the Grand Canyon to watch the sunset. I don't recall it being very expensive, but it was something different to do.
You're right about Hoover Dam - we did it in 1/2 day. Of course, DH is an engineer and he would have spent the entire day there if we let him!
Can't tell you much about other places in Arizona - DH's brother lives in LA, so we drove from the Grand Canyon to LA to visit him, and then we spent 2 days at
Disneyland/Calif. Adventure. It's an 8-hour drive from the Grand Canyon to LA/Anaheim - we broke it into 2 days, and stayed overnight at Lake Havasu City, AZ (I heard about it on the Travel Channel). Can't tell you much about Lake Havasu - the day we were there, it was 113 degrees in the shade, so we didn't leave the hotel.
Oh - driving from Vegas to the Grand Canyon, make sure you stop in Seligman, AZ and eat at the Road Kill Cafe - they have great food!! Their motto is "you kill it, and we grill it." It's on historic Route 66.
Ok, I'd better get some work done. PM me if you need more info.