CstarU: Ha Ha. Maybe I should also suggest Cocktails and Lace, or has that closed down??
EIPremier: I live in San Diego, and I really think if you have only a day you should go to the Zoo and Balboa Park. It is a really nice place with museums and parks etc. Plus it is right next to downtown and the airport!
I wouldn't recommend going to Sea World. I wouldn't prefer it over the zoo if you only had one day. If you had more time maybe.
The Hotel Del Coronado is nice, the beach there is very good too but it is probably going to be too cold to go to the beach anywhere.
So if you only have a day here, Don't listen to any of these other 'foreigners,' and go to the zoo/Balboa Park.