The Aquarium is wonderful and something I'd highly recommend.
Haven't been to the Science Center or the Fullerton Arboretum.
And the R.R. Library is really nice, although when we went we were already in Simi Valley so we didn't make a special trip out to see just that. So all we did was walk around the grounds (which are so pretty) and to see the tomb/memorial area. We didn't want to pay to see the inside when we knew the kids wouldn't really be interested.
As far as other things to do in So Cal.....well, the list is endless and really depends on your interests, ages in the group, if you are renting a car and on your budget.
Here are a few of the things we used to do when we were there:
Drive to Venice Beach and people watch. See all the crazy shops and tattoo parlors. And muscle beach.
Universal Studios in Universal City
Cruise down PCH to Malibu and check out the beaches and houses.
Go to Hollywood, see the Walk of Fame, Mann's Chinese Theater, eat at the Spaghetti Factory, maybe catch a movie at the Cinerama Dome. Or just people watch. Shop at Fredericks of Hollywood.
La Brea Tar pits
Griffith Park Observatory for one of the best views of the city lights at night.
Olvera Street for some fun shopping and some amazing taquitoes.
Legoland in Carlsbad
There is lots of shopping every where. And lots of museums.
Check out Angel's Flight, a true Los Angeles landmark.
Go to San Diego. Beautiful city with lots to do, including Sea World, Seaport Village, some great museums, more shopping, etc.
If the ages of the group are old enough it might be fun to take in a live taping of a t.v show. Tickets are usually free, but it can be time consuming.
Have a burger at In'n'Out.
There's lots to do!! Good luck and have fun.
