San Diego can easily fill an entire vacation, but its location also makes it a useful base for exploring more of Southern California. Coast, mountains and desert all sit within reach if you are willing to spend part of the day travelling.
Head north along the coast
Coastal communities north of central San Diego offer beaches, restaurants and a more relaxed change of scenery. Choose one or two stops rather than trying to collect every beach town in a single day.
Mountains and inland towns
Drive inland and the landscape changes quickly. Mountain communities can provide cooler air, hiking and a very different Southern California atmosphere.
Desert landscapes
With enough time and suitable weather, desert destinations can create the strongest contrast of all. Heat can be dangerous, particularly in warmer months, so conditions should determine the plan.
Cross-border travel
Tijuana is geographically close, but an international border crossing requires separate planning. Check current passport, entry, safety, insurance and border-wait information before considering the trip.
Do not leave San Diego every day
I would choose one day trip based on what your city itinerary lacks: another beach, mountains, desert or a different urban experience.
The goal is not to cover more miles. It is to understand how varied the landscape around San Diego really is.