More impressive than the actual exhibits is the architecture of California Science Center and just the feeling it brings. Inside and out, the California Science Center is a beautifully designed space, with an open modern feel. Everywhere you look there are interesting lines, from the kinetic folding sculpture in the lobby to the perfect rows of trees you see out the large windows. From the moment you walk up to the building, you will notice its two colorful structures, one orange, the other glass. Between them is a very high sculpture of various-sized golden balls that hang on wires- you can walk among them from one of the upper floors. 

There are many exhibits to explore. In the second-floor west wing is the World of Life area. Here you can turn a knob and watch a human digestive system's entire 18 feet unravel. You can also watch a heart transplant or drive a simulator car to compare drunk driving with sober driving. You can sit on a chair filled with cigarette butts to watch a movie on lung cancer. Kids enjoy hearing the heartbeats of different animals. Little kids will enjoy the Discovery Room where they can put on a puppet show or observe tiny frogs and mice.
Creative World, in the east wing, has some cool hands-on exhibits. Play virtual volleyball, play air drums, construct model buildings and then watch what an earthquake does to them, learn about compression by building styrofoam archways, or type in messages that will be relayed via satellite.
Forty-feet above the ground in the center of the museum is a cable wire that you can ride as if you're in a circus high-wire act. You're strapped in to a weighted bike and there's safety netting. It costs a extra to try.
In the lobby there's a rockclimbing wall that is easy for kids- my son loved it! There's also a good museum store with science kits and educational toys. We bought a balsa-wood glider plane to make on our way to the Air and Space Gallery next door. 
At the Air and Space Gallery, you can see marvelous planes of all types, in a colorful lively space. There's a replica of the 1902 Wright glider, and an F-20, all coolly suspended from the ceiling! Kids can climb into a police helicopter and pretend to use the controls. They can also put on foam wings and stand in front of a wind machine to try to fly! They can push a button to eject parachutes. 
There are some great space exhibits. You can see yourself in a mirror to see what you'd look like in space, where fluids gravitate toward your upper body- yikes! You can listen to the difference between a normal heartbeat and one of an astronaut who just returned from orbit. You can also see a full-scale version of the Cassini, which orbited Saturn, a heat shield from Gemini 11, and two space capsules. Little kids can try on astronaut clothes in a mission control room downstairs. Outside, check out the awesome SR-71 Blackbird!
Next to the Science Center, there's an IMAX theater, with seven-storey-high screen. Some of the shows are better than others.
If you're hungry, try out the MegaBites Eatery in the museum, a self-serve cafeteria. You can sit by the large windows and look out on the tree-lined pathway outside- it's very sunny, happy, and Southern-California-looking.
Walk over to the Exposition Park Rose Garden to get some nature after being indoors. The Natural History Museum is also in Exposition Park but I don't recommend it if you have young kids.
If it's the weekend, Highway 10 won't be so bad and you could drive 20 minutes to Santa Monica Beach and Pier to get in some nature after being inside the museum. It's a beautiful spot!

Directions

The museum is FREE! Some things cost money, such as the rockclimbing wall, high-wire bike, or IMAX shows. Parking is $10. Open 10-5 every day except Thanksgiving, Christmas, and New Years Day. 
California Science Center is located at 700 Exposition Park Drive, Los Angeles CA 90037. Call (323) 724-3623.
Exit the 110 Freeway southbound (or northbound not in the carpool lane) at Exposition Blvd. Turn left on Flower Street, left on Figueroa. The California Science Center entrance (Exposition Park) will be immediately on your right.
Traffic will be very bad unless it's the weekend.

Click on map for interactive view