Resident here. I agree with Jackson about the beach towns. Coronado has the spectacular bonus of the bridge which takes you there and back. Along the coast road, you'll pass UC San Diego, the Salk Institute (architecture) and Torrey Pines GC. Veering inland can take you through wealthy Rancho Santa Fe, which gives you that side of San Diego.
I detest downtown, where I lived for two years and have worked for 20+ years; recommend a walk through nearby Balboa Park, which also includes the zoo. Beside it is Bankers Hill, an old neighborhood which merits a stroll. Not far away is Mission Hills, another of the city's older neighborhoods; it has pretty streets and homes.
The county's "Backcountry" has many worthwhile rural destinations. If interested, I can elaborate.
My current taste runs in different direction: gritty National City. I doubt it is for you.