To me, the coolest thing about San Diego is the amazing variety of the geology, the botany, the climates, and the fauna.
To fully capture this diversity, I would devote one day to a driving survey of San Diego County. You will see ocean, lagoons, suburbia, inland valleys, mountains, and desert. The trip is mostly secondary roads and is about 250 miles (depending on some options). It will be a decidedly un-city trip. There are numerous camping possibilities en route.
I would start from the ocean head inland on CA-76/79/78. Return via CA-86 and I-8.
I recommend the “Explore! San Diego County” Map from AAA. If you can’t get that the other standard map is the Thomas Bros. Map book but that will set you back about $20.
If something like this interests you, send me email and I’ll give you a lot of details and suggested stopping points.