C.Y.
I have a few suggestions...
1) Try and find an organized playgroup near you. You can try http://www.meetup.com/ (just type in "Playgroup" or "Stay At Home Mom" along with your zip code to find groups in your area). Also check out http://www.momsclub.org. They have local chapters in nearly every city and get together several times a week with their children.
2) Check out the storytime's in your area, either at the public library or at your local Borders Bookstore. It is easy to meet other moms with young kids there, to create your own playgroup to plan activities with. Parks are great for this too.
3) There are a couple of great books to give you ideas about what to do with your children. "Baby's Day Out In Southern California: Fun Places to Go With Babies and Toddlers" is by JoBea Holt. "Fun Places to Go With Kids in Southern California" by Susan Peterson is also great. Both have tons of ideas of places to go that are fun, educational, and low cost or free!
4) Check out My Gym (http://www.my-gym.com/) as an alternative to Gymboree. My Gym is geared for kids up to 9 years of age -- there is one in La Verne, which isn't too far from you.
Hope these suggestions are helpful. Good for you for making the decision to stay home with your kids. These early years are so very important, and they go by in a flash. Good luck!