That can be a rather long day although it is very do-able. But if you have time to go less miles and play more along the way, that's always a good thing.
You might enjoy going east on I-40 and then dipping down to Lake Havasu City to see London Bridge. That's about 230 miles so would make a good halfway point. You could leave by going south on 95 until Quartzite. Quartzite is an amazing place where people by the hundreds (maybe thousands?) boondock their RVs. You might find it interesting but, unfortunately, the RVs are there in the winter so you will be missing that sight. Then go west on I-10 to Box Canyon Road, then south on Hwy 86 which will take you by the Salton Sea and you can take one of several routes through Anza-Borrego Desert State Park, then drive through Cuyamacho Rancho State Park, and catch I-8 into San Diego.
Now, I've never driven these roads myself so I can't give specific recommendations based on experience. But, on a map, it looks like a terrific drive. Hopefully someone will pop in here with experience to weigh in on this idea.
This whole drive is about 580 miles from Grand Canyon to San Diego.