IF you are asking about the weather("what will it be like") then it is similar to LA.
December will usually have temps in the 60sF during the day and can reach 70F if you are lucky! At night the temps will be lower. It can also be rainy at that time of year but since we don't get much rain(usually) then it shouldn't bother you.
It will be off season for prices at the beach so you may find a good deal!
There are many other things to see/do besides the zoo but it depends on what interests you. There are ships to visit on the bayfront downtown.
The USS Midway is an aircraft carrier and our Maritime Museum has a Russian sub and old sailing vessels to explore.
There's Balboa Park (where the zoo is located) which has small museums/gardens/live theater/artisan shops etc.
You might also consider our La Jolla neighborhood(for hotels as well)
which has the cove with its seals and sea birds in their native environment, upscale boutiques and art galleries/restaurants etc.
There's also Cabrillo National Monument with its 180 deg. views/Ca. history,/old lighthouse.
There are other things to do here too but you can't see them all in such a short visit!!
Depending on the dates you are here there may be boat parades on the bays and the December Nights event in Balboa Park.