I apparently live in the only part of California I can stand, since I don't like heat. The Bay Area is relatively cool, the rest of California is hot. San Diego is doubly so. Everyone needs hats, the bigger the brim the better. Sunscreen and a water bottle are a must.
San Diego Zoo vs Safari Park
With limited time and some free coupons in hand we hit the Safari park instead of the zoo, which is out in a part of the Gobi desert known as "Escondido". It's very hot and sunny there, hydrate often, bring your big hat and your walking shoes. Free coupons won't get you much except the basic tram tour, and the wait (in the blazing sun and heat) is over an hour for it since it's free.
There are plenty of animals but they're very spread out. You'll do more walking than animal appreciation unless you drop $300-$400 on the special safari tour where you go in a bus out into the special no-pedestrian-access area and feed giraffes leaves from within your safari jeep. If you have that kind of money, go for it, it sounds like a once-in-a-lifetime experience.
Legoland is not owned by the Lego corporation, it's owned by Merlin Entertainment. It's actually quite a fun park for little kids. You won't find any adult thrill rides here, everyting has been scaled down in perceived risk for kids of the 3-13 age group. If your kid doesn't like Legos, the park is about half as big, since there's a lot of Lego-focused activities. There's the test, build and race room, where you build a racer out of Legos and run it against the other kids. There's also many places where you play with Legos or look at enormous models of Legos.
Go on a weekday and if you're clever, spend the morning in the pool and arrive at Legoland at 2pm. It's open until 8pm and you'll get only the coolest parts of the day there.
There's a special deal where you can add a second day to your Legoland ticket for $10 per person. That's a really good deal, go back for a second day from 2pm 8pm.
Do I need to explain that even though the park closes at 8pm, the store at the entrance stays open a very long time after closing?
The pool / the place to stay
I don't really know how anyone survives a vacation in July and August in San Diego without access to a pool. I spent many an hour floating in the one at our "resort", the Carslbad Seapointe Resort.
Speaking of which, this is an entirely satisfactory place to stay. We waited too long to make plans and all that was available was this place near Legoland. We got an apartment with a full kitchen, a separate bedroom, and a couch bed for the kids. We made almost all our meals and that recouped some of the cost of the room over what we would have paid for a cheaper place without a kitchen and eating out all the time.