I'm an LA native, and I'd really have to think about what to suggest because it really depends upon what you're into.
Since you're into amusement parks, then Disneyland is really a good one to visit. I really wasn't bowled over by Universal Studios the one time I went (and I was a theatre student who went with a bunch of my theatre student friends). Knott's is okay. Magic Mountain I've heard is really full of gangbangers these days (although I have to admit that I'm planning my first trip there in 20+ years to ride the Colossus one last time). Even as an adult (with no children), I did greatly enjoy having a meal at Goofy's Kitchen since the characters always went there (and, frankly, I was on a kick for buffets with real mashed potatoes). There's a restaurant in Disneyland in New Orleans Square that's a nice restaurant, but you need reservations. I believe it's called "Blue Bayou."
I think I did whale-watching once or twice as a field trip or two for school and really wasn't into them. You might want to incorporate a trip to Catalina since you'll be doing that.
If you plan on visiting the beach, Santa Monica Pier and Venice Beach are cool (but I really loved Santa Monica before they created Third Street Promenade).
As some others have suggested, a studio tour could be fun. If you really like Disney, seeing a movie at the El Capitan is pretty awesome as Disney owns it (and it's across the street from where they have all the hands and feet in cement).
Good on deciding to rent a car. You really can't get around LA efficiently without one (and the public transportation can be quite nasty -- like smelling-full-of-urine nasty).
I love going to the museums so the Getty is good, as is LACMA (LA County Museum of Art). Mind you, there are two Getty's -- one is off 405 and the other is in Pacific Palisades (I believe the Pacific Palisades one has antiquities). My favorite is the Huntington Library and Museum in San Marino.
If you want to go to one of the touristy spots to eat, Jerry's Deli in Studio City was known as the place where a lot of the Hollywood power players met (I don't know if it still is). Philippe's, as someone suggested, is good. Olvera Street is close by and a fun cultural location (and some good Mexican food could be found). Little Tokyo is also in the vicinity and is also pretty cool.