There's two places that come to mind when thinking of amusement parks and LA, Disneyland and Universal Studios. Both are excellent amusement parks and are well worth visiting over and over again.
There's another fun theme park to visit and that Knotts Berry Farm, otherwise known as KBF. There's lots of fun things to do at KBF, from all the great rides, fun amusement games, exciting shopping and wonderful food.
In October the park is transformed to Knott's Scary Farm and its the place to be if you like Haunted Houses. This event happens every night after the park closes and is for adults only. Its so popular, that tickets sells out on weekends and some weeknights!
The other thing I really like about KBF is the food. From the Turkey Legs to Fried dough, there's lots of good food to enjoy in the park. My personal favorite is the Fried Chicken at the Mrs. Knott's Chicken Dinner Restaurant just outside the park. What a great way to end the day!
If your looking for another place to visit when in the LA area, you should seriously try Knotts Berry Farm. Download this KBF PDF to show you all the great things you can find at Knotts Berry Farm.