SpaceShipOne.
Of course, with only one seat, I'd have to fly, but with a little training I think I could have a go. I'd be leaving from and returning to Mojave, but probably seeing everything along the way. Give it a few years though, because I'd rather fly SpaceShipThree.
The Grand Canyon is amazing from the air. I'd also say Crater Lake and Mt St. Helens are possibilities.