Los Angeles (and the LA area) is a very popular destination. When most people think of visiting LA they think of Hollywood, Disneyland, or the beach. We'll cover each of those:
Hollywood
Hollywood is not the glamorous place people probably picture. While there are some nice spots, the main part of Hollywood is actually not very nice.
Amusement Parks
There are several major amusement parks in the LA area. Disneyland (which is South of Los Angeles in Anaheim), Knotts Berry Farm, and Universal Studios are the biggest attractions.
Disneyland now actually has 2 separate parks, Disneyland and California Adventure. They are both a lot of fun and I'd recommend a day at both. California Adventure is quite a bit smaller and, especially if the crowds aren't bad, you can make it through all of California Adventure easily in a day. They do have some great, unique rides. One favorite is Soaring Over California, where you have the sensation of riding a glider over various scenery around California. There is also an excellent roller coaster called California Screamin'. Overall I'm not sure if I could really recommend California Adventure though, the price is quite a bit higher than some other parks in the area, like Knotts Berry Farm, without really offering much to make it worth it.
Knotts Berry Farm is an often overlooked park because Disneyland is so famous, but Knotts is actually the first amusment park in America. They have a lot of rides, and a lot more variety in their rides, than Disneyland. Knotts is really a fun park and the price can be half of what you would pay at Disneyland. Before you go, print your own tickets on line and you can save significantly. You can buy tickets here.
Disneyland, of course, is a classic, but if you have been to Disney World in Florida then be prepared for much, much smaller park.
Beaches
Beaches in Southern California are not like beaches in the Caribbean or Hawaii. The water is colder, the sand isn't as fine, and some beaches are dirty. That said, there are still some good beach experiences to be had as long as you go in with accurate expectations. Some of the most popular beaches are Zuma Beach (north of Malibu), Manhattan Beach, Venice Beach, Paradise Cove, Santa Monica Beach.