Welcome aboard the RoadTrip America Forums!
While I applaud your desire to keep your passengers entertained on this trip, I don't think just doling out bits of trivia will do it. It's a short enough drive between San Diego and Tucson that you can actually make a few stops that will be both enjoyable and enlightening. Places to consider include the
Desert View Tower in Jacumba for an initial bit of wackiness; the old
Yuma Prison which is a surprisingly good spot for a lunch break with picnic tables overlooking the Colorado River; the store in Dateland, AZ that has great shakes including a
date flavored one for which it is famous;
Casa Grande National Monument near the junction of I-8 and I-10; and two attractions near Picacho Peak north of Tucson:
Arizona's Civil War Battlefield and an
Ostrich Farm. You can split those up and see some on the way to Tucson and some on the way home, but as we say around here, There are no boring roads.
AZBuck