You have some friends or family coming to visit. Or perhaps its a long weekend but you can get away long enough to make plane trip worth it. What can you do? Well, there are a number of places near San Diego but just far enough away to warrant a day trip. (Or even a whole weekend)
1.) Julien. VERY small rustic town, locally famous for its apple pies! Off I-8.
2.) Jacumba Hot Springs.
3.) Glenn Ivy Spa.
4.) Anza Borrego Dessert.
5) Casa Grande. Small town off I-8 just before you hit the 10 intersection towards Phoenix. Native American Ruins and a great Belgian Waffle Place.
6.) San Diego Train Museum.
7.) Mexico. Tijuana is literally across San Diego's southern border, and can be done in less than a day (many undergrads go clubbing down there to avoid the 21+ restrictions, to give a sense how easy it is). Certainly going further in is a worthwhile trip, however, as most of the best of what Mexico has to offer lies beyond TJ.