It's very strange: you go down into the basement of what looks like an office building and there is a children's play center. However it has its advantages in that if it is raining it becomes a great place to go (just get there early as it gets crowded). Habitot is small but full of fun for little ones: a full sized space ship for them to explore, a water play area, a crawlers' play zone, face painting and more!
One visit can get veru pricey, but if you buy a membership it pays itself off in 2 visits. It is also very convenient to BART so you do not have to live close by to take advantage of this great resource.