San Diego is one of the nicest cities in the country. It's a major tourist destination and there are tons of things to do.
Mission and Pacific Beach are your best bet for fun in the sun. Belmont Park has a vintage wooden roller coaster (one of two remaining seaside coasters on the west coast) as well as other rides. There is a strand (boardwalk) that runs the entire 3 mile length of both beaches. It's a great place to walk, run, rent a bike or skates.
http://www.missionbeachonline.com/
Some must see attractions are the world famous Zoo, Sea World, and Balboa Park. Balboa Park is the largest urban cultural park in the country. You can get a park passport which allows you entry to a dozen museums as well as the zoo.
http://www.balboapark.org/info/passport.php
Other attractions in the park are the Air and Space Museum, Ruben H. Fleet Science Center, the Natural History Museum, the Museum of Man, the Model Rauilroad Museum, the Automotive Museum, the Old Globe Theatre, the Hall of Champions, the Botanical Gardens, and the largest outdoor pipe organ in the world (free concerts on Sunday).
North County San Diego is home to the Zoo's Safari Park. There are multi acre enclosures where herds of wild animals roam plus lots of hiking trails. It's worth the visit and is a different experience from the zoo. You can ride a zip line, tour the park on a Segway, or take a photo caravan into the enclosures and feed the giraffes.
http://www.sdzsafaripark.org/
La Jolla offers great views, great surfing, great food, great theater, and great shopping.
Check out the surfers at historic Windansea Beach. At low tide you can walk out onto the tide pools. Walk along the Cove and check out the caves. Visit the Museum of Contemporary Art. Take a stroll on Prospect St and check out the shops, galleries, and have some authentic Mexican food at Alfonso's. Then head to the Birch Aquarium at The Scripps Institution of Oceanography. It has a spectacular view of the bluffs over the Pacific Ocean. The Tony award winning La Jolla Playhouse is located nearby on the campus of UCSD. For a few laughs at night, The Comedy Store is on Pearl St.
http://aquarium.ucsd.edu/
Take the trolley from Downtown to Old Town. There is a state park there that recreates life in the Mexican and early American periods of 1821 to 1872.
The Old Point Loma Lighthouse at the Cabrillo National Monument offers great views of the downtown skyline, San Diego Harbor and the Pacific Ocean. On a clear day you can see all the way to Mexico. The spring is a great time to go whale watching as they migrate along the coast.
The USS Midway is an aircraft carrier turned into a museum
http://www.midway.org/
For a great dinner with the best view of the downtown skyline, go to Pehoe's in Coronado. You can take the water taxi across the harbor.
http://www.peohes.com/
La Jolla offers some great placesc to eat. For Mexican food at it's Alfonso's. More down to earth is Jose's Courtroom where the locals hang out to drink and eat. The Spot offers Chicago style food with both deep dish and thin crust pizza. For gourmet cuisine, it's George's at the Cove.
The Mandarin House in La Jolla features home made noodles and some of the best Chinese food in town. It's located one block from the world famous Windansea Beach. Check out the surfers and take a sunset stroll. Then have dinner.
Closer to downtown and one of my favorites is El Indio. Serving Mexican food since 1940, it's a short walk from the Washington St. trolley stop. It's where the taquito was invented.
http://www.el-indio.com/
Kansas City Barbeque is located at the Seaport Village trolley stop. Best BBQ in town, inexpensive, and it's where they filmed the bar scene in Top Gun. Plenty of interesting memorabilia on the walls and it also has outside dining.
Old Town has one of the best Italian restaurants in town. Pizza Bella features home made Italian food made from Mama Bella's recipes. During the day their courtyard is open for outdoor dining. It's one block from the Old Town trolley stop.
Check out these sites for discount coupons and other info
http://www.sandiego.org/nav/Visitors
http://thebestplacesinsandiego.com/