When in San Diego, try Burger Lounge (5 locations). Way better than the overhyped In-N-Out, in my opinion, but more expensive. ($7.95 for the burger alone, but worth it). Mexican food could almost be considered southern California's specialty cuisine, there is a taco shop almost on every block, a sit-down Mexican restaurant in every neighborhood, and so many choices of where to get it that you should study yelp.com in advance to find some that people are recommending in the area you will be at.
California has it all... beaches, mountains, deserts, farmlands, cities, towns, culture, history, sports, theme parks, whatever you want, it's probably somewhere in California. There's so much variety and such great quantity that it's hard to choose. It really depends on what interests you.
Personally I believe the 3 best things in California are Yosemite Valley, Point Lobos State Natural Reserve and Disneyland. I'm sure others will argue and there are many other good things to see and do, but those are the three world-class-level sights California has.
You could consider a loop drive for a few days which would take you north from the LA area to Yosemite, spend a full day in Yosemite Valley, then drive west over to the coast, spend a few hours at Point Lobos, then drive the spectacular Highway 1 south from there through the Big Sur country, then go back to southern California. Yosemite Valley can be reached in about a 7-hour drive from Buena Park.
Julia Pfeiffer Burns State Park in Big Sur has a gorgeous cove with a waterfall that lands on the beach (though you can't go down to the beach, you can look at it from a platform). If you pay the day use parking fee at one state park or reserve, it is valid for all other state parks or reserves on the same day. The 90 miles of Highway 1 from Carmel to San Simeon is one of the world's great coastal drives. I think Yosemite Valley is California's best feature and should be visited by everyone at least once. The whole trip could be done in 4 to 5 days.
There is plenty of snow in the mountains right now, if skiing or snowboarding interest you. The San Bernardino National Forest has 3 ski areas (Snow Valley, Snow Summit and Bear Mountain) within about a 2-hour drive of Buena Park. The Palm Springs Aerial Tramway takes you from the desert floor to the top of Mount San Jacinto, currently a winter wonderland of snow-clad forest at the top. Wildflowers are starting to come into bloom in the deserts. Anza-Borrego State Park east of San Diego, and Joshua Tree National Park north of Palm Springs are two good places for wildflowers in the spring. The drive from San Diego to Mount Laguna, along Sunrise Highway to Julian (a historic town now known for apple pie) and down into Anza-Borrego is a great drive. At this time of year it's possible to go surfing in the morning (with a wet suit), drive one hour and go cross-country skiing at Mount Laguna, then drive another hour and go dirt-biking in the desert. No other place in the world is that possible in such a short time that I know of. Death Valley National Park is another possibility, though that is quite a way out of the way. Grand Canyon National Park is one of the wonders of the natural world and is a day's drive from southern California.
If dirt biking and off-roading is your thing, head out to Glamis, 3 hours east of San Diego, a huge area of sand dunes with thousands and thousands of people riding all over them every weekend. There are other such areas in the deserts, but that's the biggest "mecca." You can rent the vehicles there too, although I have no idea how much it costs. If you're interested in ocean activities, you can take deep-sea fishing trips from Dana Point in southern Orange County, or it's not too late for a whale-watching cruise if that interests you.
I think one of the best tourist attractions in southern California is the Huntington Library, at San Marino, near Pasadena. Besides the library (Gutenberg Bible) and the art gallery (Gainsborough's "The Blue Boy") they also have - and I think it is the main draw - 120 acres of gardens in 14 themes. It is a beautiful place, and it will probably be starting to come into spring bloom now. It is only $15 on weekdays, but their hours are short, like noon to 4:30. For more details see
http://www.huntington.org/If theme parks interest you, there is Disneyland and Disney California Adventure, Universal Studios Hollywood, Knott's Berry Farm (right there in Buena Park) and Six Flags Magic Mountain, which has the most big roller coasters and thrill rides. In fact, when the 2 new ones there are open this summer, Magic Mountain will have the most roller coasters of any theme park in the world. This is a very good time to go to those parks, since they are uncrowded and lines will be short.
In San Diego, you may want to visit La Jolla Cove, Torrey Pines State Reserve and Cabrillo National Monument are all worth your time, and are either free or low-cost. Cabrillo is only $5 for the car and has spectacular views, historical exhibits, an old lighthouse, and tidepools to explore at low tide. Of course the San Diego Zoo, the San Diego Safari Park, and Sea World. If you like animals, I think the San Diego Zoo is a must-see. If you go there, go first thing in the morning when the animals are more active. There are also many museums nearby, and some gardens which are free. The Botanical Building (featured briefly in "Citizen Kane") is free and has rare orchids and other tropical plants.
San Diego's "signature dish" is the fish taco, and there are many good places to get them, but one of the best is actually bar very close to the ocean, the South Beach Bar & Grille in the Ocean Beach neighborhood of San Diego. Ocean Beach is the "hippie" neighborhood of generally-conservative San Diego, the neighborhood where DUers would meet if they had a meetup, the place where someone is probably buying a bong at The Black or a Doors album on vinyl at one of the record stores at this moment. The South Beach Bar & Grille is the last business on Newport Avenue before the beach starts. It is very crowded during the happy hour when the food is cheaper, so I would go there for lunch. They accept cash only! But the fish tacos are outstanding, and they always have several beers you have never heard of on special that day. After eating there, take a stroll on the Ocean Beach pier, the longest fishing pier on the west coast.
There's much more in San Diego, the USS Midway Museum which is a decommissioned aircraft carrier, the Maritime Museum, which is a collection of ships including 2 submarines and the Star of India, the world's oldest working ship. There are many good beaches in southern California. I think some of the prettiest and best are Windansea Beach in La Jolla (the beach made famous by Tom Wolfe's book "The Pump House Gang," Coast Boulevard Park in La Jolla, La Jolla Cove, the famous Black's Beach, formerly (and still unofficially) a nude beach, north of La Jolla, the huge and relatively unspoiled San Onofre State Beach near San Clemente, and the beautiful, unspoiled Crystal Cove State Park near Laguna Beach.
Close to Buena Park, you might visit the Tucker Wildlife Sanctuary in the hille east of Orange (my hometown) and the large, old Irvine Regional Park also in those same hills. There is a lot more to see and do in Orange County besides the Disney parks and Knott's and the beaches. For more on Orange County, see
http://www.visittheoc.com/ and
http://www.orangecounty.net/html/visitor.html For places to eat, you'll have to rely on local recommendations and sites such as
http://yelp.com ,
http://chowhound.com and
http://tripadvisor.comEnjoy your California vacation!