Hi Soy,
San Diego is a great city...much more laid back than L.A. or San Francisco, a lovely place to visit.
We had a great holiday there in Sept 98 but stayed with relations so can't recommend accommodation. I agree with all Sheelaghs suggestions of places to visit. Tijuana is worth a trip (across the border in Mexico) but a bit dodgy..wouldn't fancy being there after dark & I agree not to bring children.
You should visit Coronado. It's a suburb of San Diego on a peninsula jutting out into the Pacific. We went by boat which was lovely. Coronado is very picturesque and and also houses the hotel where 'Some Like It Hot' was filmed, I loved that film. The hotel backs on to a beautiful beach (where all the beach scenes with Marilyn Monroe & Tony Curtis were filmed) so bring your togs.
We also went to a baseball game in the Qualcom Stadium. Know nothing about baseball but had a ball as the atmosphere was terrific.
Have a great trip.