Everybody knows about the Zoo and the beaches in San Diego County. Here's a more comprehensive list of things to see/do in this area:
The most well-known:
The San Diego Zoo – one of the world's best zoos. The animals are in simulated habitats like the ones they would live in the wild. You view them over a berm, through plexiglass, or other enclosure. The only “caged” animals are in the Snake and Reptile house. You walk, take a tour bus, and/or glide over the park in a skyride. The only downside to the zoo is that it's not even close to free. Plan to spend a day there.
The San Diego Safari Park (formerly the San Diego Wild Animal Park) – is the sister park to the Zoo. Located in the northern end of San Diego County (east of Escondido), this is a big expansive zoo. There are less animals but more space for them to live in. See this park by walking, take a ride around it, or (extra $) a safari into the African expanse. Another park that isn't close to free, but like the Zoo, it does have some reciprocity with other zoos. If you are a zoo member in your home community, check the zoo's website to see if you can get a discount upon entry.
Sea World San Diego – one of several of these over the country. This is one of the originals. You can spend a day here seeing whale, dolphin and seal shows, feeding sea creatures (extra $), strolling through aquariums and past pools with many types of different sea creatures. You can also ride a few rides and spend some time eating (and waiting for shows). Price of admission is similar to any theme park.
Beaches – San Diego County hosts several. There are state and city park beaches. Mission Beach is popular, as is the beach at Del Mar and Torrey Pines State Beach. If you are so inclined, we have the infamous Black's Beach (on maps, it's right next to/part of Torrey Pines City Beach; it's a nude beach). One of the most famous beaches is by the Hotel del Coronado, across the Coronado Bay Bridge.
Birch Aquarium – a research aquarium with many types of sea life. Plan to spend a half day. Reasonable admission. The aquarium hosts concerts over the warm season – outdoors. Birch is part of the Scripps Institute of Oceanography, which is part of UCSD.
Glider Port – Not far from the Birch Aquarium is the Salk Institute and the Glider Port nearby. You can sit on the top of the bluffs and watch hang-gliders. Free!
Old Town State Park – Spend a delightful historical morning checking out life in San Diego in the early 1800's by visiting Old Town State Park. There are displays, recreations, and museums, even a schoolhouse! Then have lunch at a nearby Mexican restaurant – there are some very good ones adjacent to Old Town.
Shopping – Horton Plaza and Seaport Village are the closest to downtown San Diego. If you park in Horton Plaza's garage, make a note of what fruit your garage is named after, or you won't find your car later!
US Navy – no doubt about it, San Diego has a very strong military presence. We have many Navy and Marines Corps bases here. You won't gain access to any of them without an ID card, but if you want to see a Navy ship, go to Navy Pier downtown where the US Midway (an old carrier) is permanently docked. You can take a tour. If you like ships, walk a little north of there to the Maritime Museum and check out the ships there.
Hot days – sometimes San Diego can get pretty hot. We have a water park (The Wave in Vista, and another one (Knott's Soak City in Chula Vista.
Want to hike? Mission Trails Regional Park, just west of Santee, has trails of interest, as does Los Penasquitos Canyon Preserve just west of Poway.
Missions – There is a long trail of 21 missions through the state of California. Two of them are within the borders of San Diego County. Mission San Diego de Alcala is the one closest to downtown San Diego. Up near Oceanside, there is Mission San Luis Rey. Nearby is a third, San Juan Capistrano. If you are familiar with the legend of the swallows at Capistrano, this is THAT mission.