We stayed within walking distance of Balboa Park, and spent two full days there--and we chose not to see the Zoo. We took the bus to see the USS Midway (wonderful--allow at least four hours), a harbor boat tour, and a self-guided walking tour of the Gaslamp District.

We didn't have a car, and didn't need one for what we did. If we had wanted to see some less easily accessible areas, I'd have rented a car for a day or two.