I visited San Diego in May on a whirlwind overnighter with my son's class (5th and 6th grades). I don't know what you prefer to do/see, but I suggest you either write to the Chamber of Commerce, contact AAA or try online for the city's website (not entirely sure they even have one, but have a hunch). A lot depends on how much $$ you want to spend.
Here's a recap of what WE did in two days (includes free critique, based on my personal tastes):
Cabrillo Monument and Beach (tidepools); Tidepools neet, took a small walk up to the site of the first lighthouse, on a point near the museum. That was kinda cool, to see how the folks lived who inhabited the lighthouse (it's all fixed up to display period furniture in each of several rooms). That is all free.
San Diego Mission (Mission San Diego de Alcala, which was named a minor basilica by the Pope in 1976); went on the docent-guided tour (they needed a more kid-and-bored-adult-friendly docent for our group!!). I happen to enjoy old buildings and their stories, so I enjoyed it all. The kids, when polled at the end of the trip, hailed this stop as their favorite -- WHEN IT ENDED!!!
Old Town; again, old buildings from the beginnings of San Diego. Grab a map. Check out Whaley House (oldest brick structure in S.D., supposed to be haunted; it was closed by the time we reached it so we couldn't go in). We walked up San Diego Ave (away from Old Town), and ate dinner at a Old Town Mexican Cafe (opposite side of street from Whaley House) that served HAND-MADE tortillas (could watch em being formed/heated from the street, in the front window). Yummy!!
Seaport Village; wandered around shops/restaurants. If you like Ben and Jerry's ice cream, they have an ice cream shop in the Village.
San Diego Natural History Museum; choice of movies included w/admission. When we visited the two were: Ocean Oasis and When Dinosaurs Roamed America (w/selections from the Discovery Channel special). Don't know what the featured exhibit will be; check out www.sdnhm.org.
Check out the HUGE spreading old tree in front of the museum.
San Diego Zoo; always interesting (my boys love the Monitor lizard and the Komodo dragon) - don't know if Mei, the Panda baby born at the zoo, will have gone (back to China) by then; parent pandas still at San Diego. Enjoyed hearing stories about tricks the elephants play on visitors, out of boredom. A lot of the animals are given a new toy or toys every day, to keep them occupied and happy.
The Zoo and Natural History Museum are in Balboa Park which has a lot to see, just in and of itself. The Coronado Hotel (or Hotel Coronado) is the one mentioned in the above post. Seen pix, never been there. Look up the Old Spaghetti Factory for one of your meals. If you've never been, go. If you have been, and like it (!) there is one in San Diego. If you've been and don't like it, don't go, suit yourself (so profound).
Sea World, as other posters have mentioned, is a must to visit, if you have the funds. Great shows/exhibits. I don't think one could see it all in one day . . . Sorry I don't know more about family-friendly beaches. Perhaps another poster will help you there. Haven't been other than the above and one other time, so haven't spent leisurely beach exploring time. There is a trolley line (Red Trolley, I believe) that takes you all over the main areas of San Diego, for not much $$. If you want a different trip, take the Amtrak Coaster up the coast to some of the other beaches (Carlsbad, Oceanside, . . .). Don't hafta drive!
Everyone I have talked to who has been to San Diego LOVES it, all say something good about its climate, have heard more than one person sigh and say they'd like to live/retire there . . .
If you like ghost stories, see if you can locate hmmm . . . . a tour of supposedly haunted houses. You're picked up in a bus converted to look like a hearse, driver wearing black clothes and stovepipe hat, carries lantern. (tour goes during day or nite, your choice). To old part of cemetary, where driver tells stories of persons buried there, then on to 2 or 3 old houses with stories behind each one . . . one is the Whaley House, where sometimes the docent joins in with personal anecdotes. The writer of the article I read said it was the BEST money she and daughter had spent there, thoroughly enjoyed it. Sorry, don't remember the name of the tour. . . I'll see what I can find on the web.
Sorry so long post. PM if you want more. Have a good time!! -- D