You won't regret going to the Grand Canyon. Book a helicopter trip: expensive but worth it IMO. It's hard to appreciate the size of it, just looking down from the rim.

If you haven't seen it, seek out the Sam Willis series in BBC TV about the 're-creation' of John Wesley Powell's exploration of the Grand Canyon.
https://www.youtube.com/watch?v=p3r3g16Xl-A

Powell was an amazing character!