I asked for suggestions of places where to take my 3 yo when we're visiting her grandparents in the Los Angeles area. These are some of the suggestions we've gotten. If you know of other places, please make a comment.
You'll see that this list doesn't include the LA Zoo - my parents took her during her last visit and they were very disappointed. There were very few animals and these were far away from each other.
The Long Beach Aquarium
Santa Monica Pier
Train Museum in Griffith park
Kids Zone @ 4th and Broadway (indoor play area)
Huntington Botanical Gardens
Descanso Gardens
Exposition Park