Non bonsai related but Balboa park in San Diego is great. Tons of stuff to do between the zoo and museums, I'd definitely spend a little time there (I love the aviation museum). Also, while in SD, find yourself a good California burrito, best in the world. If you are beach people there are tons of options. Ocean beach is the hippy beach, you'll always find people in drum circles or twirling fire. Cool vibe and very laid back little downtown area. Pacific Beach and Mission beach are more touristy but a nice little boardwalk to walk up and down. And if you want something quieter head up to La Jolla or Del Mar. Also, if time permits and you're a baseball fan there is no better park than Petco.
Also, I'm sure you've been warned but expect traffic. The drive from SD To Santa Monica could take as little as 2 hours or as much as 4-5 depending on time of day.
As far as bonsai,
@Ruddigger hit the main ones. Huntington Library is beautiful and you could easily spend a whole day there between all of the non-bonsai gardens and bonsai/penjing displays.
Are you looking to buy some significant trees while you are out here? I'll second Ruddigger, David Nguy's trees are unreal, his junipers are incredible.