If you have a baby or toddler, Blossom Birth Services in Palo Alto is great. They offer lots of classes, groups, etc.
Also, check out the Palo Alto/Menlo Park Mother's Club (PAMP). Their website is a good starting place. It is a huge group that isn't necessarily AP, but they have all kinds of "sub-clubs" that may be of interest to you.
Also, make sure you check out the Palo Alto Junior Museum and Zoo - it's in the same area as a playground, children's library and children's theater. Oh, and they offer some great classes for kids.