Definitely go to Disneyland (I love it even after having gone there at least 20 times), visit the new J. Paul Getty Museum, Walk down Universal City Walk, go to the beach, eat in Chinatown ... The list is unending!
Quick Tips:
Visiting Los Angeles for the first time is probably an intimidating experience. Plan all your major activities in advance, including transportation, to make your stay less stressful.
Less touristy places can also be fun, so be open to asking for ideas from locals.
Best Way To Get Around:
Do you remember that song from the 80's with the lyrics, "Nobody walks in LA..."? Well, that's pretty much true, unless you count the business types who go out during lunch breaks wearing suits and sneakers.
Renting a car is probably the best way to go. Public transportation is an option, but I don't have personal experience with it, so I can't say whether its a good choice or not.