I wish we had gone to the Hoover Dam while we were there this summer, as we had our car and have never been. But I couldn't pry hubby away from the slots and I didn't want to go alone. But it's quite the thing to drive over, just a huge operation, and I think it would be interesting. Plus, I've always wanted to take a dam tour, a la National Lampoon's Family Vacation.
Also, Bryce Canyon, if you are going to venture that far north, is one of the most spectacular places we've ever been. I found it much more fascinating than the Grand Canyon. The Grand Canyon is, well, grand, it's big and awe inspiring. With the Bryce Canyon area, there's something new and beautiful to look at around every corner. But that would definitely be an overnight trip at least.
THose are my two votes.