You won't miss much if you don't pass through Yuma. I live here, and there is very little to see, although the sand dunes where they filmed Star Wars (or one of the original 3 movies) are nearby. Plus, the desert, and it is hot, and will be hot.

Markoff mentioned the 4 corners, and this is near Monument Valley and Canyon de Chelly. Plus, southern Utah and the parks there are worthwhile. Route 66, the bits still in use, are west of Flagstaff, and would be worth a ride that way towards California.

A sort of route is visualized; start in Phoenix, head up through the Arizona mountains through Holbrook, and north into the Navajo Reservation to the Four Corners, west to either Canyon de Chelly or Monument Valley, but do both, whichever seems the quicker to get to, and into southern Utah, back down to see the Grand Canyon, and through Flagstaff (a Route 66 town) westward on Route 66, and some of the little towns on R 66 such as Williams, Seligman, etc.

Then into California, either on Interstate 40 to gain some time as there is not much to see from Kingman to Barstow, although Lake Havasu might be a quick diversion/break between them. I don't know which way you desire to head once into California, but Barstow is a good crossroads to either head straight south to San Diego, not so south to Los Angeles, or continue west to the Central Coast of California.

Keep in mind, from Phoenix to Barstow can be done in 3 to 5 days, or stretched to a week, so you still have another 11 weeks to get more places in, and others will give more recommendations from there. I wish to join you on the northern Arizona route, but I don't think my work schedule will allow it. I used to live near the 4 corners area, so do know it fairly well. Either that, or when you loop back through Arizona later on.