I agree that the B'hai temple in Wilmette, just up Sheridan Road from Chicago going north, is one of the most beautiful structures in the United States.

Further West, the two National Parks I love the best are Bryce Canyon (The epitome of Red Canyonland) and Zion National Park, both in Utah.

If you are going through Arizona, there is a simply magical drive from just north of Tucson that takes you through Sedona and up to Flagstaff. It's through Oak Creek Canyon, and one of nature's wonders.

North of Albequerque is Sante Fe, still one of the most beautiful towns in the West...and from there, it's a straight shoot up through the Rockies.

If you stay more south (I just remembered you're going to San Diego, sorry I got carried away with all the natural beauty there still is in the US, and I guess I'm a wee bit homesick) just South of San Diego is La hoya (I might be spelling that wrong). If you ever wanted to know what Greece looks like, the rocks and coastline around there are dead ringers.

Have a terrific trip!..xx..e

(PS-I don't know if they still do it, but AAA used to arrange individual "triptychs" for people, taking them through the most beautiful routes from one destination to another. I've used them in the past and been utterly delighted. Also, their maps clearly show which are the best routes through a state for sight-seeing. Great maps, and it used to be quite inexpensive to join, and well-worth it to be able to use their roadside pickup emergency facilities, etc)

(PPS-Did I say I envy you? I used to criss cross the states regularly just for the fun of it, (sadly, I haven't done it for over a decade) and took a different route each time. You'll be wishing you had longer to do what you're doing, I promise you.)