This trip (my fourth), we were only there for a day so we spent it at the Disneyland Park. It was the first time I had seen Downtown Disney, which is an exciting strip of stores and restaurants that connects Disneyland and Disney's California Adventure (which we did not have time to see). We can't wait to go back when we have more time to see it all!
It was the first time I got to experience the use of the park’s "fast pass" -- this is a way of avoiding standing in lines for the more popular rides. If it is an extremely busy day, I would definitely recommend getting your fast pass time for Splash Mountain and Indiana Jones early. We went mid-afternoon to get our time for Splash Mountain and did not get on until almost midnight. (The park was open late the day we were there.)
Food is very expensive in the park, so be prepared or do what we saw others doing -- they packed lunches for the family in rolling coolers.