In the ridge-top parks that form a green belt from San Pablo to Castro Valley, you will find open spaces that cater to a greater population density. While they can sometimes be crowded, these parks--tucked into canyons and hidden valleys--can often feel way more remote from the urban centers below than they really are. There are a lot of great activities for the littlest hikers in some of these parks: farms, zoos, pools, playgrounds, picnic areas, and trains.