San Diego County is at the southern tip of California. Known for great weather, San Diego also has a lot to offer.
Beaches
With the warm weather going to the beach is a popular thing to do in San Diego. Keep in mind, however, that the water in San Diego is not warm like it is in Hawaii or the Caribbean. You can swim and there is a lot of good surfing, but there are not any tropical reefs for snorkeling or scuba diving. Two of the nicest beaches are La Jolla Cove, where you can see seals lounging on the sand, and Coronado where the famous Hotel Del Coronodo sits. Another great stop is Cabrillo National Monument and the Tide Pools. Before You Go find out when the low tide will be and plan your visit accordingly. You'll be able to walk around the tide pools and see a lot of fish, crabs, and maybe even octopus.
Amusement Parks and Zoos
San Diego is home to Legoland California, Sea World, the World Famous San Diego Zoo and the Wild Animal Park. A few thoughts on each:
Legoland California: Legoland is fun, but it is not huge. Although it was built fairly recently it already looks pretty old and a lot of colors have faded. It is worth a trip, but if you do not make it it's also not a big regret.
Sea World: Sea World is a lot of fun, featuring entertaining shows, a few good rides, and a lot of great animal exhibits. Well worth a visit.
San Diego Zoo: The San Diego Zoo is a big zoo with a lot of great animal exhibits. I would not say there is anything that really sets it apart from other zoos in a huge way, but if you like zoos then this is a good one.
Wild Animal Park: The Wild Animal Park, in northern San Diego county, is set up more like a huge area that combines lots of animals that would usually be found together in Africa. You take a train around that area and there are also some other more typical zoo exhibits. The Wild Animal Park is unique and has some good shows, especially their bird show, and for zoo fans it is well worth a trip.
Museums
Near the San Diego Zoo in downtown San Diego is Balboa Park. There are up to 18 museums in Balboa Park, depending on your definition of a museum. There are a few large ones, such as the Reuben H. Fleet Science Center, the San Diego Air & Space Museum, the San Diego Museum of Man, and the Timken Museum of Art. Aside from the museums, Balboa Park is also just a great place to walk around and see the architecture and gardens.
Downtown Area
Balboa Park and the San Diego Zoo are both basically in the downtown area. Not quite walking distance but very close. Downtown San Diego also has some other nice places to walk around and things to see. You can take a harbor cruise, there are some outside shopping areas like Horton Plaza (a mall) and Seaport Village, which is a nice area to walk around right next to the water and browse shops.