The Grand Canyon of course..
But aside from that obvious landmark, simply driving south down the middle of AZ on I-17 is worth the trip alone, IMO. That is a gorgeous drive, and Phoenix (although hot) is a pretty cool city. Flagstaff is also good. Meteor Crater, just off I-40 due east of Flagstaff (about 30-45 min., if I recall), the list goes on. I'm rather envious of Matt, as I only got to spend a short time there, he actually gets to live there for a while!