Long Beach Aquarium, not too far south of LA is pretty nifty, but I bet the aquarium at Monterey is even more interesting. Monterey, Point Lobos, and the Pacific Coast Highway are must do.

A bit out of your way, but if you can make it to Huntington Gardens at Pasadena, they are fabulous. The Ghetty Museum in LA is also very nice, with a lovely garden to boot. Marina Del Ray is a great place to gawk at fancy sailboats.

Also fabulous, in Los Angeles, are the buskers along the pedestrian mall in Santa Monica. (At least on a weekend night.) Venice Beach is worth strolling just for the show. There is also a Buddhist center in Pacific Palisades with lovely gardens. I forget the name, but as best I recall, it is on Sunset Blvd. shortly before the blvd. drops down to the coastal highway.